PC 49, plus one in enclosed space (4)
I believe the answer is:
Here is my best explanation:
'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 heavily and wearily", "Walk slowly and heavily", "Walk heavily, trudge", "Trudge - policeman", "Walk with effort", "Walk slowly with a heavy tread", "Copper", "Walk heavily (like a policeman?)", "Trudge, walk heavily" and "Walk doggedly".)