Make fun of Middlesbrough’s river, we hear (5)
I believe the answer is:
tease
'make fun of' is the definition.
(to make fun of someone is to tease them)
'middlesbrough's river we hear' is the wordplay.
'middlesbrough's river' becomes 'Tees' (River Tees).
'we hear' indicates a 'sounds like' (homophone) clue.
'tees' is a homophone of 'TEASE'.
(Other definitions for tease that I've seen before include "Tantalize" , "Comb to remove tangles and open out the fibres" , "Use a comb" , "Kid" , "Page of book" .)