Sink's blocked here? He'll provide a plug? (5,5)
I believe the answer is:
press agent
'he'll provide a plug?' is the definition.
(a plug is a type of advertisement)
'sink's blocked here?' is the wordplay.
'sink' becomes 'sag' (sagging is a kind of sinking).
'blocked' is an insertion indicator (to block can mean to box in or contain).
'here?' becomes 'present' ('present' can be similar in meaning to 'here').
'sag' placed within 'present' is 'PRESS-AGENT'.
(Other definitions for press agent that I've seen before include "PR person" , "PA" , "publicist" , "Person seeking favourable publicity on behalf a client" .)