PC 49, plus one in enclosed space (4)
I believe the answer is:
plod
'pc' is the definition.
(informal term for a police officer)
'49 plus one in enclosed space' is the wordplay.
'49 plus one' becomes 'l' (Roman numeral for fifty).
'in' indicates putting letters inside.
'enclosed space' becomes 'pod'.
'l' going into 'pod' is 'PLOD'.
(Other definitions for plod that I've seen before include "Walk slowly with a heavy tread" , "Walk doggedly" , "Walk heavily (like a policeman?)" , "Stomp" , "Walk slowly and heavily" .)