Solution to 22A has been deliberately left unsolved and is to be answered by a non-regular/novice commenter, with proper annotation. Those who have answered earlier in the week, please give others a chance.
Open for anyone to answer, if not solved by 1 PM.
ACROSS
1 Deteriorated in speech in 10 years (6) DECADE (~decayed)
4 Say, transport cat in mat (6) CARPET {CAR}{PET}
9 Ingredients in muesli, milk are insufficient (4) SLIM [T]
10 Pastor's feeling bitter about offering (10) PRESENTING {P}{RESENTING}
12 Tied up a new thief in Delhi, held every second (8) ANCHORED {A}{N}{CHOR}{hElD}
13 Join volunteers in treat (9) ENTERTAIN {ENTER}{TA}{IN}
15 Good points on birds (5) GEESE {G}{E}{E}{S}{E}
18 Two characters substitute each offer to get dressmakers as clients (9) CUSTOMERS {C)-o+u)UST(-u+o)OMERS} each offer or each other?
22 Brussels' tie with one at Norway? No, it's Danish, perhaps (8) ?U?O?E?N (Addendum - EUROPEAN {EU}{ROPE}{A}{N} - See comments)
23 Tour without pressure, fly perhaps (6) INSECT INSpECT
25 Companies have function in public transports crossing limits at the terminus (10) {BU{SINE}SES} over {l...tS}
26 One time in elders' house there were rodents (4) RATS {R{A}{T}S}
27 Companionless ladies surround single with ponytail (6) LONELY {L}{ONE}{L}{ponY}
28 Doctor stays behind small yard having sold perimeter away to Mark (6) SYMBOL {MB}<=>{S}{Y} and {sOLd}
DOWN
3 Store in store? Yes in Spain, set inside (7) DEPOSIT {DEPO{SI}T}
5 Department in New York seizes cocaine after a great engagement at the front (6) AGENCY {N{C}Y}<=>{A}{Great}{En...t}
6 Convert rupee to ml of fuel (9) PETROLEUM*
7 When females becomes tense filters become passages (7) TUNNELS (-f+t)TUNNELS
8 Auntie's scones are prepared with perseverance (13) TENACIOUSNESS*
14 Once poison is administered after injecting some lead, it will be an outbreak (9) EXPLOSION {EX}{POISON+Lead}*
17 One French-American seen with unionist gangster which is rare (7) UNUSUAL {UN}{US}{U}{AL}
19 Longing for a number featuring Sinead O' Connor primarily (7) THIRSTY {THIR{Si...r}TY}
20 Report? Article is being rewritten (7) RECITAL*
21 Say, Rice's first conflict on East is bona fide (6) CEREAL {Co...t}{E}{REAL}
24 Supreme Court obstacle for Bush (5) SCRUB {SC}{RUB}
Reference List
Pastor = P, Street = ST, Square = S, New = N, Thief in Delhi = CHOR, Volunteers = TA(Territorial Army), Good = G, Points = E,S, Prince = P, Pressure = P, Time = T, Elders' house = RS(Rajya Sabha), Lady = L, Small = S, Yar = Y, Charlie = C, Henry = H, Society = S, Yes in Spanish = SI, Cocaine = C, Females = F, Tense = T, One in French = UN, Gangster = AL(Capone), East = E