Physical work's rejected in pen (8)
I believe the answer is:
corporal
'physical' is the definition.
('corporal' can be similar in meaning to 'physical')
'work's rejected in pen' is the wordplay.
'work' becomes 'op' (abbreviation for opus).
'rejected' says the letters should be written backwards.
'in' is an insertion indicator.
'pen' becomes 'corral' (corral is a kind of pen**).
'op' backwards is 'po'.
'po' put into 'corral' is 'CORPORAL'.
(Other definitions for corporal that I've seen before include "One trained for engagement" , "soldier?" , "Poor Carl (anag)" , "Napoleon was this little" , "NCO rank below sergeant" .)