Vendor includes river fish (5)
I believe the answer is:
trout
'fish' is the definition.
(I know that trout is a type of fish)
'vendor includes river' is the wordplay.
'vendor' becomes 'tout' (tout is a kind of vendor).
'includes' means one lot of letters goes inside another.
'river' becomes 'r' (abbreviation for river).
'tout' placed around 'r' is 'TROUT'.
(Other definitions for trout that I've seen before include "Small salmonlike fish" , "Schubert work; fish" , "one may be hooked" , "unpleasant old woman" , "Rainbow, for example" .)