Making the mistake of giving the cockney fish (6)
I believe the answer is:
erring
'the cockney fish' is the definition.
('herring' - herring is a type of fish - said with a cockney accent, dropping the 'H')
'making the mistake of' is the wordplay.
I cannot quite see how this works, but
'mistake' could be 'err' (to err is to make a mistake or blunder) and 'err' is found within the answer.
This explanation may well be incorrect...
'giving' is the link.
Can you help me to learn more?
(Other definitions for erring that I've seen before include "taking wrong direction" , "Sinful" , "Making a mistake" , "Being incorrect" , "Going wrong" .)