Overturned river barge contains head of rotting fish (8)
I believe the answer is:
mackerel
'fish' is the definition.
(mackerel is a kind of fish)
'overturned river barge contains head of rotting' is the wordplay.
'overturned' shows that the letters should be reversed in order.
'river' becomes 'cam' (river in Cambridge, England).
'barge' becomes 'keel' (both are a type of boat).
'contains' is an insertion indicator.
'head of' says to take the initial letters.
The first letter of 'rotting' is 'r'.
'cam' written backwards gives 'mac'.
'keel' placed around 'r' is 'kerel'.
'mac'+'kerel'='MACKEREL'
(Other definitions for mackerel that I've seen before include "Commonly-eaten fish" , "A fish; sort of breeze, sky" , "Common food fish" , "Important food fish" , "Marine food-fish" .)