Measure used by the police once more (5)
I believe the answer is:
metre
'measure' is the definition.
(I know that measure can be written as metre)
'by the police once more' is the wordplay.
'by' is a charade indicator (letters next to each other).
'the police' becomes 'met' (London Metropolitan Police).
'once more' becomes 're' (prefix in words like 're-emerge' or 're-enter').
'met'+'re' is 'METRE'.
'used' acts as a link.
(Other definitions for metre that I've seen before include "measure of length" , "39.37 inches" , "Approx 39 inches" , "Approx. thirty nine point three seven inches" , "determined by scanning" .)