Casual trousers, almost loose (5)
I believe the answer is:
slack
'loose' is the definition.
(synonyms)
'casual trousers almost' is the wordplay.
'casual trousers' becomes 'slacks' (I've seen this before).
'almost' means to remove the last letter (most of the word but not all of it).
'slacks' with its final letter removed is 'SLACK'.
(Other definitions for slack that I've seen before include "bits of coal" , "coal dust" , "Lax" , "Lazy and careless" , "Not taut" .)