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 "Throw angrily in spell of indulgent pleasure", "Brief relationship", "Toss", "Brief affair; throw", "Throw, cast", "Throw - short affair", "Throw; Scottish dance", "Highland dance - spell of way-out behaviour", "Launch or propel" and "Throw as in Highland dance?".)