Casual trousers, almost loose (5)
I believe the answer is:
slack
'loose' is the definition.
(slacking is a kind of loosing)
'casual trousers almost' is the wordplay.
'casual trousers' becomes 'slacks' (I've seen this before).
'almost' means to remove the last letter (almost the whole word is kept).
'slacks' with its final letter taken off is 'SLACK'.
(Other definitions for slack that I've seen before include "bits of coal" , "coal dust" , "Lax" , "Lazy and careless" , "Not taut" .)