What holds policeman back? Age (5)
I believe the answer is:
epoch
'age' is the definition.
(I've seen this before)
'what holds policeman back?' is the wordplay.
'what' becomes 'eh' (sound used to ask 'what?' or 'pardon?').
'holds' is an insertion indicator.
'policeman' becomes 'cop' (slang term).
'back?' says the letters should be written backwards (I've seen 'backed' mean this).
'cop' written backwards gives 'poc'.
'eh' enclosing 'poc' is 'EPOCH'.
(Other definitions for epoch that I've seen before include "Major division or period of history" , "Sure date" , "Historical era" , "A great age from which dates are reckoned" , "Division of a geological period" .)