Solution to 18A 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 solve, if not solved by 1 PM.
ACROSS
1 Recent communication about a missing Earl (6) LATTER {L{A}eTTER}
4 Agitated swimmer eating constantly (8) FEVERISH {F{EVER}ISH}
9 Soccer balls near goalpost's end grabbed by linesman from back (6) FOOTER {OO}{g...sT} in {REF<=}
10 Difficulty in speech... and I capitulate (8) HANDICAP [T]
11 Things are stirring in images of that old European complex (3,4,2,5) THE GAME IS AFOOT {IMAGES+OF+THAT+O+E}*
13 Practical test that is included by banks for financial director ahead of court proceeding (5,5) FIELD TRIAL {Fi...{IE}...aL}{D}{TRIAL}
14 Check by spinning top of tuner on TV (4) TEST {Tu..r}{SET}<=
16 Unevenly rough, dry part of Germany (4) RUHR {RoUgH+dRy}
18 Spies taken in by extremely bright discharged emigrants who frequent upper-class dos (10) ?O?I?L?T?S ()
21 A mantis flirting with sinister marsupial (9,5) TASMANIAN DEVIL {A+MANTIS}*{AND}{EVIL}
26 "Dash" in special font (6) SPRINT {S}{PRINT}
DOWN
1 Pick up fight after changing sides (4) LIFT (-r+l)LIFT
3 Nuclei of cells modified in degrees after removing electron (8) ENERGIDS {IN+DEGReES}*
5 In the morning, Avtaar visits former kingdom for investigation (11) EXAMINATION {AM}{I} in {EX}{NATION}
6 Get close to target and rip top off (6) ENDEAR {END}{tEAR}
7 Go down supporting popular Conservative ring (7) INCLOSE {LOSE}<=>{IN}{C}
8 Condition is almost pathetic after treatment (9) HEPATITIS {IS+PATHETIc}*
12 Likeness of, say, ruler with gold skirt reflecting on casing of gemstone (6,5) MIRROR IMAGE {AMIR}{OR}{RIM}<={Ge...nE}
13 Eldest brother is new head of family after he goes abroad (5-4) FIRST-BORN {BROTheR+IS+N+Fa...y}*
15 Cool to follow book with cryptic clue of top quality (4-4) BLUE CHIP {HIP}<=>{B}{CLUE*}
17 Touch of titanium and silver below pot# (7) HASHTAG {Ti,,,m}{AG}<=>{HASH}
19 Stability's in doubt - possibly stay away, after leaving son in capital (7) TBILISI staBILITyS Third I from? Seems to be an error in fodder?
20 Cryptically, it's savage (6) SADIST {ITS}* [RA]
22 Superman's well-known place in England (4) KENT [DD]
Reference List
Earl = E, Old = O, European = E, Director = D, River = R, Special = S, Children = CH, Electron = E, Conservative = C, Gold = OR, New = N, Book = B