Baggy line that's lining bag (5)
I believe the answer is:
slack
'baggy' is the definition.
(thesaurus)
'line that's lining bag' is the wordplay.
'line' becomes 'l' (used when specifying particular lines from a poem).
'that's lining' means one lot of letters goes inside another.
'bag' becomes 'sack' (sack is a kind of bag).
'l' inserted inside 'sack' is 'SLACK'.
(Other definitions for slack that I've seen before include "Loose" , "Not taut" , "bits of coal" , "Idle and disinclined to work" , "Powdery coal" .)