Don't know why but found it tough going today.
ACROSS
9 Every other concert has to be at this time of the year (7) OCTOBER {
10 Till he joins a union (7) HUSBAND [DD]
11 I must exercise — forced or by force (7) IMPETUS {I}{M{PE}TUS*}
12 Avoid part of the composition (7) REFRAIN [DD]
13 Polls giving license to corrupt (9) ELECTIONS*
15 Going over a plot of land showing interest in plant (5) ERICA {ER{I}CA}<=
16 Get worse but gain about fiftytwo points (7) RELAPSE
19 With two constants, let’s modify letter (7) EPISTLE {E}{PI}{LETS*}
20 Negative review — in all fairness unimpressive (5) MINUS [T<=]
21 Stopping tax on land is pointless (9) CESSATION {CESS}{
25 Excited player will press on, it’s swell (7) UPSURGE
26 Both sides have a stake over northern light (7) LANTERN {L}{ANTE}{R}{N}
28 One had made mistake among scholars (7) ACADEME {AC{MADE*}E}
29 Agitated soldiers try to run away (7) RESTIVE {RE}{ST
DOWN
1 I eat after ten… (6) IODINE {10}{DINE}
2 … and the main ingredient is coreless apple, by the way (6) STAPLE {ST}{AP
3 Notice has no right to circulate (4) OBIT O
4 Outside, not a large painting (6) FRESCO
5 Period covering jail term, enough initially for the sanctimonious fellow (8) PHARISEE {PHA{RI}SE}{E}
6 Prepared a feast, do I include a flavouring? (10) ASAFOETIDA {AS{A}FOETIDA*}
7 Horrible stint as a devil worshipper (8) SATANIST*
8 Sir Anthony on gallery, lacking bite (8) EDENTATE {EDEN}{TATE}
14 Attempts to follow records of paintings (10) TAPESTRIES {TAPES}{TRIES}
16 Butter, doubtful with a fruit (8) RAMBUTAN {RAM}{BUT}{AN} A relative of the Lychee
17 Sports official’s marginalised (8) LINESMAN [CD]
18 She married outside college, after sweetheart shunned (8) ESCHEWED {
22 Wage hike after a year sadly (6) SALARY {SALA}{RY}<=
23 Ex Soviet secret police chief following one to Spain (6) IBERIA {1}{BERIA}
24 Article sent away to a place in France (6) NANTES {AN+SENT}*
27 Home is truly where the heart is (4) NEST