Ring a policeman about to incarcerate (4)
I believe the answer is:
coop
'incarcerate' is the definition.
(thesaurus)
'ring a policeman about' is the wordplay.
'ring' becomes 'o' (**).
'a policeman' becomes 'cop' (cop is a kind of policeman).
'about' is an insertion indicator.
'o' put into 'cop' is 'COOP'.
'to' acts as a link.
(Other definitions for coop that I've seen before include "pound" , "Cage for poultry" , "Store" , "Pen" , "box of chickens" .)