Mentioned a bit of a fork in river (4)
I believe the answer is:
tyne
'river' is the definition.
(I've seen this before)
'mentioned a bit of a fork' is the wordplay.
'mentioned' shows a homophone (sound like).
'a bit of a fork' becomes 'tine' ('tine' is part of 'fork').
'tine' is a homophone of 'TYNE'.
'in' acts as a link.
(Other definitions for tyne that I've seen before include "River running through Newcastle" , "River of NE England" , "North of England river" , "Geordie river" , "Banker" .)