Initially baggy rear in evidence for some pants (6)
I believe the answer is:
briefs
'pants' is the definition.
(thesaurus)
'initially baggy rear in evidence for some' is the wordplay.
'initially' says to take the initial letters.
The initial letters of 'baggy rear in evidence for some' is 'BRIEFS'.
(Other definitions for briefs that I've seen before include "Abstracts" , "Digests" , "CLOTHING" , "Instructs a barrister" , "Close-fitting underpants" .)