Decline to participate in getting behind travel agents around start of summer (7)
I believe the answer is:
abstain
'decline to participate' is the definition.
'in getting behind travel agents around start of summer' is the wordplay.
'getting behind' means one lot of letters go next to another (some letters go behind others).
'travel agents' becomes 'abta' (Association of British Travel Agents).
'around' means one lot of letters goes inside another.
'start of' indicates taking the first letters.
The initial letter of 'summer' is 's'.
'abta' enclosing 's' is 'absta'.
'in' put after 'absta' is 'ABSTAIN'.
(Other definitions for abstain that I've seen before include "decline at voting time" , "Choose not to do something" , "Don't indulge" , "Refrain from doing something" , "Give up smoking, say" .)