Harass peacekeepers boarding carrier (5)
I believe the answer is:
hound
'harass' is the definition.
(thesaurus)
'peacekeepers boarding carrier' is the wordplay.
'peacekeepers' becomes 'un'.
'boarding' is an insertion indicator.
'carrier' becomes 'hod' (I've seen this before).
'un' inserted within 'hod' is 'HOUND'.
(Other definitions for hound that I've seen before include "Persecute; dog" , "Pursue relentlessly" , "badger" , "Harry" , "Give no peace" .)