Lambert occupies swell place to sleep in USA (4)
I believe the answer is:
flop
'place to sleep in usa' is the definition.
(US term for a cheap bed or resting place)
'lambert occupies swell' is the wordplay.
'lambert' becomes 'l' ('L' can be a synonym of 'lambert').
'occupies' indicates putting letters inside.
'swell' becomes 'fop' ('fop' can be a synonym of 'swell').
'l' placed inside 'fop' is 'FLOP'.
(Other definitions for flop that I've seen before include "Failure - loll" , "Collapse loosely - total failure" , "Dud - bomb" , "Fall limply - box-office failure" , "Fall clumsily, liked failed play" .)