Sum up days lost by policeman (5)
I believe the answer is:
recap
'sum up' is the definition.
(recapping is a kind of summing up)
'days lost by policeman' is the wordplay.
'days' becomes 'd'.
'lost by' is a deletion indicator.
'policeman' becomes 'redcap' (nickname for member of the Royal Military Police).
'redcap' with 'd' taken out is 'RECAP'.
(Other definitions for recap that I've seen before include "Final summary" , "Run over the points again" , "sum everything up" , "In brief, say it again" , "Go back over the main points" .)