Officer's hat strap one's broken (7)
I believe the answer is:
captain
'officer's' is the definition.
(captain is a kind of officer)
'hat strap one's broken' is the wordplay.
'hat' becomes 'cap' (I've seen this before).
'strap' becomes 'tan' (both can mean to hit as punishment).
'one' becomes 'i' (Roman numeral).
'broken' is an insertion indicator.
'tan' placed around 'i' is 'tain'.
'cap'+'tain'='CAPTAIN'
(Other definitions for captain that I've seen before include "Ship's commander" , "eg Hornblower" , "Head of a ship's crew" , "Is Pat Cain in charge of the ship?" , "Skipper" .)