Attached to group in the beginning (5)
I believe the answer is:
onset
'the beginning' is the definition.
(onset is a kind of beginning)
'attached to group' is the wordplay.
'attached' becomes 'on' (I've seen this in other clues).
'to group' becomes 'set' (both can mean a collection).
'on'+'set'='ONSET'
'in' acts as a link.
(Other definitions for onset that I've seen before include "Start, beginning" , "origin" , "Attack using stone" , "Assault; beginning" , "Beginning; violent attack" .)