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 subsidiary indication.
'49 plus one' becomes 'l' (Roman numeral for fifty).
'in' indicates putting letters inside.
'enclosed space' becomes 'pod'.
'l' inserted into 'pod' is 'plod'.
(Other definitions for plod that I've seen before include "Tread wearily", "Walk slowly and heavily", "Walk with effort", "Trudge, walk heavily", "Walk heavily (like a policeman?)", "Walk slowly with a heavy tread", "Walk heavily, trudge", "Trudge wearily", "Make slow progress" and "Shamus, say".)