Man on wagon in AA meeting (7)
I believe the answer is:
regatta
'meeting' is the definition.
(I've seen this before)
'man on wagon in aa' is the wordplay.
'man' becomes 'reg' (short for Reginald).
'on wagon' becomes 'tt' (I've seen this in other clues).
'in' indicates putting letters inside.
'tt' put within 'aa' is 'atta'.
'reg'+'atta'='REGATTA'
(Other definitions for regatta that I've seen before include "which may involve eights" , "Yachting or rowing event" , "Aquatic activity" , "Craft meeting" , "Organised boat races" .)