Sit quietly and lose pounds less! (4)
I believe the answer is:
pose
'sit' is the definition.
(as in sitting for a painter)
'quietly and lose pounds less' is the wordplay.
'quietly' becomes 'p' (p for piano and quiet).
'and' says to put letters next to each other.
'pounds' becomes 'L' (abbreviation e.g. Lsd - pounds, shillings, pence).
'less' is a deletion indicator.
'lose' with 'l' removed is 'ose'.
'p'+'ose'='POSE'
(Other definitions for pose that I've seen before include "Arrange, model" , "Sit; attitude" , "position" , "Affectation" , "Sit for picture" .)