Having coat on boat, catching river fish (8)
I believe the answer is:
mackerel
'fish' is the definition.
(mackerel is a kind of fish)
'having coat on boat catching river' is the wordplay.
I cannot quite see how this works, but
'coat' could be 'mack' (mack is a kind of coat) and 'mack' is found in the answer.
'on' could be 're' ('on' can be mean 'with reference to') and 're' is found within the answer.
'boat' could be 'e' and 'e' is found in the answer.
A single letter 'l' remains which might be clued in a way I don't see.
This may be the basis of clue (or it may be nonsense).
(Other definitions for mackerel that I've seen before include "Fish caught by spinner (or sprat?)" , "Swimmer" , "One in school" , "Spiny-finned food fish" , "Common striped fish, very tasty" .)