Fellow dallied not half in affair (5)
I believe the answer is:
Here is my best explanation:
'affair' is the definition.
'fellow dallied not half' is the subsidiary indication.
'fellow' becomes 'f' (abbreviation - of a society etc.).
'dallied' becomes 'lingered' ('linger' can be a synonym of 'dally').
'not half' suggests halving the letters.
'lingered' halved 'ling'.
'in' is the link.
(Other definitions for fling that I've seen before include "Launch or propel", "Dance", "good time", "Throw angrily in spell of indulgent pleasure", "Hurl", "Throw violently", "Throw - short affair", "quick one?", "knees-up at 8 [HIGHLAND GAMES]" and "brief relationship".)