Rubbish produced by a fish's blow (10)
I believe the answer is:
codswallop
'rubbish' is the definition.
(I've seen this before)
'by a fish's blow' is the wordplay.
'by' is a charade indicator (letters next to each other).
'a fish's' becomes 'cods' (cod is a kind of fish).
'blow' becomes 'wallop' (wallop is a kind of blow).
'cods' put next to 'wallop' is 'CODSWALLOP'.
'produced' acts as a link.
(Other definitions for codswallop that I've seen before include "waffle?" , "tripe" , "Rubbish!" , "Nonsense (colloq.)" , "[BUNKUM]" .)