Conflict in group over head, mostly (3-2)
I believe the answer is:
set-to
'conflict' is the definition.
(set-to is a kind of conflict)
'group over head mostly' is the wordplay.
'group over' becomes 'set' (I can't explain this - if you can you should give a lot more credence to this answer).
'head' becomes 'top' ('top' can be a synonym of 'head').
'mostly' means to remove the last letter.
'top' with its final letter taken off is 'to'.
'set'+'to'='SET-TO'
'in' acts as a link.
(Other definitions for set-to that I've seen before include "brush with violence?" , "Disagreement, fight" , "Scrap" , "Brief fight" , "Altercation" .)