Fish with musical tail (4)
I believe the answer is:
coda
'musical tail' is the definition.
(I've seen this in another clue)
'fish' is the wordplay.
I cannot really see how this works, but
'fish' could be 'cod' (type of fish) and 'cod' is found within the answer.
A single letter 'a' remains which might be clued in a way I don't understand.
This explanation may well be incorrect...
'with' is the link.
Can you help me to learn more?
(Other definitions for coda that I've seen before include "Ending of musical piece" , "Closing part of a musical composition" , "Closing section of a musical composition" , "Concluding section" , "Cocluding musical passage" .)