Mock pronunciation of river (5)
I believe the answer is:
tease
'mock' is the definition.
(teasing is a kind of mocking)
'pronunciation of river' is the wordplay.
'pronunciation of' shows a homophone (sound like).
'river' becomes 'tees' (River Tees).
'tees' is a homophone of 'TEASE'.
(Other definitions for tease that I've seen before include "Use a comb" , "Chaff" , "Banter, make gentle fun of" , "Poke fun at" , "Rag" .)