Measure taken by police and engineers (5)
I believe the answer is:
metre
'measure' is the definition.
(I know that measure can be written as metre)
'police and engineers' is the wordplay.
'police' becomes 'met' (London Metropolitan Police).
'and' means one lot of letters go next to another.
'engineers' becomes 're' (Royal Engineers).
'met'+'re'='METRE'
'taken by' acts as a link.
(Other definitions for metre that I've seen before include "determined by scanning" , "measure of length" , "Unit of length in metric system" , "Approx 39 inches" , "100cm" .)