Policeman turning up clutching a hat (3)
I believe the answer is:
cap
'hat' is the definition.
(thesaurus)
'policeman turning up clutching a' is the wordplay.
'policeman' becomes 'pc' (**).
'turning up' says the letters should be written in reverse.
'clutching' indicates putting letters inside.
'pc' in reverse letter order is 'cp'.
'cp' enclosing 'a' is 'CAP'.
(Other definitions for cap that I've seen before include "Limit (prices)" , "Upper limit; hat" , "'If the . . . fits, wear it' (3" , "Covering for head or bottle" , "Surpass with headwear" .)