Loose bit of lettuce put in bag (5)
I believe the answer is:
slack
'loose' is the definition.
(I know that loose can be written as slack)
'bit of lettuce put in bag' is the wordplay.
'bit of' indicates taking the first letters.
'put in' is an insertion indicator.
'bag' becomes 'sack' (both can mean a container).
The first letter of 'lettuce' is 'l'.
'l' placed within 'sack' is 'SLACK'.
(Other definitions for slack that I've seen before include "Flaccid" , "Not taut" , "Lax" , "Powdery coal" , "skive" .)