Sleep on, good man? (4)
I believe the answer is:
rest
'sleep' is the definition.
('rest' can be a synonym of 'sleep')
'on good man?' is the wordplay.
'on' becomes 're' ('on' can be mean 'with reference to').
'good man?' becomes 'st'.
're'+'st'='REST'
(Other definitions for rest that I've seen before include "'''God ... ye, merry gentlemen'' (carol) (4)'" , "Relaxation" , "Settle" , "Repose - remainder" , "Take it easy, have a break" .)