Solution to 17D has been deliberately left unsolved and is to be answered only 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 Suggest plan - "50% to quit for change of government" (6) PUTSCH {PUT}{SCHeme}
4 Bar or joint mostly housing animals from the east (8) OBSTACLE {EL{CATS}BOw}<=}
10 Boss to lead with articles (3,6) TOP BANANA {TOP B}{AN}{AN}{A}
12 Thrill from fifteenth US expedition (7) ENTHUSE [T]
13 Rebuffed most of adamant boy's prayers (7) ORISONS {IROn<=}{SONS}
14 Voila! Cricket has turned around (5) CIRCA [T<=]
15 Spreadsheet not set up, somehow given a new form (8) RESHAPED {sPREADSHEet}*
18 Spooner's carried food for grandfather, say (8) FOREBEAR (~bore fare to forebear}
20 Force European to take over (5) BARGE {E}{GRAB}<=
23 Cuts sticks (7) CLEAVES [DD]
25 Caught in difficulty, ring for investigator (7) CORONER {COR{O}NER}
26 Guides leaderless groups (5) RINGS bRINGS
27 River trip enthrals old relative (3,6) RIO GRANDE {RI{O}{GRAN}DE}
28 Each must vault in ancient sacrifice (8) HECATOMB {EACH}*{TOMB}
29 Prohibits Spain's recurring problems (6) ESTOPS {ES}{SPOT<=}
DOWN
1 Cool head when tackling nice problem (8) PATIENCE {PAT{NICE*}E} Semi&lit
2 Bartender feeding drop of port to expert sampler (7) TAPSTER {TA{Port}STER}
3 Ordered catalogue set (9) COAGULATE*
5 Merry Corsican tourist on vacation kidnapped by barbarian - a slippery killer (3,11) BOA CONSTRICTOR {CORSICAN+To...sT}* in {BOOR}}
7 Conductor lost hat in program (7) CATHODE {C{HAT*}ODE}
8 Path of exit for one on ship (6) EGRESS {EG}{RE}{SS}
9 Obey corrupt food marketers pinching recipe (4,6,4) TAKE ORDERS FROM {FOOD+MARKETERS}* over {R}
16 Emir brags about component of perfume (9) AMBERGRIS*
19 Marine in nice coat nicked rum (7) OCEANIC {NICE+COAt}*
21 Meet heads of rebellious trade union at work (3,4) RUN INTO {Re...s+Tr..e+UNION}*
22 Burn my school bags (6) SCORCH {S{COR}CH}
24 Stop very sceptical question (5) VISIT {V}{IS IT}
Reference List
European = E, Old = O, Spain = ES, For one = EG, On = RE, Ship = SS, Recipe = R, Fellow = F, My = COR, School = SCH, Very = V