Keep going when officer lags (4)
I believe the answer is:
last
'keep going' is the definition.
(thesaurus)
'when officer lags' is the wordplay.
'when' becomes 'as'.
'officer' becomes 'lt' (I've seen this in other clues).
'lags' indicates putting letters inside (to lag can mean to provide covering or insulation).
'as' put within 'lt' is 'LAST'.
(Other definitions for last that I've seen before include "Bringing up the rear" , "Endure - this clue is!" , "Survive; after all others" , "Shoemaker's model" , "Final; keep going" .)