John noticed detective has unfinished business (5,4)
I believe the answer is:
loose ends
'unfinished business' is the definition.
(I've seen this before)
'john noticed detective' is the wordplay.
'john' becomes 'loo' (both can mean a toilet).
'noticed' becomes 'seen' (I've seen this before).
'detective' becomes 'DS' (detective sergeant).
'loo'+'seen'+'ds'='LOOSE ENDS'
'has' acts as a link.
(Other definitions for loose ends that I've seen before include "Unfinished business" , "details yet to be explained" , "items to be settled" .)