Sit lazily (6)
I believe the answer is:
slouch
'sit lazily' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for slouch that I've seen before include "Loll" , "Sit lazily" , "Walk in an ungainly drooping manner" , "Sit with poor posture" , "Lazy and drooping posture" .)