Fellow dallied not half in affair (5)
I believe the answer is:
fling
'affair' is the definition.
(I have seen 'Brief affair' mean 'fling' so perhaps 'affair' could also mean 'fling')
'fellow dallied not half' is the wordplay.
'fellow' becomes 'f' (abbreviation - of a society etc.).
'dallied' becomes 'lingered' ('linger' can be a synonym of 'dally').
'not half' indicates taking half.
'lingered' with half the letters taken is 'ling'.
'f'+'ling'='FLING'
'in' is the link.
(Other definitions for fling that I've seen before include "knees-up at 8 [HIGHLAND GAMES]" , "Hurl" , "Toss" , "See 7" , "Throw angrily in spell of indulgent pleasure" .)