Mostly opted to hem trendy trousers (6)
I believe the answer is:
chinos
'trousers' is the definition.
(chinos are a type of trousers)
'mostly opted to hem trendy' is the wordplay.
'mostly' means to remove the last letter (most but not all of the word).
'opted' becomes 'chose' (I've seen this in another clue).
'to hem' indicates putting letters inside (hem can mean to enclose).
'trendy' becomes 'in' ('in' can mean fashionable).
'chose' with its last letter taken away is 'chos'.
'chos' enclosing 'in' is 'CHINOS'.
(Other definitions for chinos that I've seen before include "Trousers made from a twilled cotton" , "Casual cotton trousers" .)