Urchin knocked back fish and a toasted cake (10)
I believe the answer is:
ragamuffin
'urchin' is the definition.
(I know that ragamuffin is a type of urchin)
'knocked back fish and a toasted cake' is the wordplay.
'knocked back' shows that the letters should be reversed in order.
'fish' becomes 'gar' (type of gar).
'and' means one lot of letters go next to another.
'toasted cake' becomes 'muffin' (I've seen this before).
'gar' written backwards gives 'rag'.
'rag'+'a'+'muffin'='RAGAMUFFIN'
(Other definitions for ragamuffin that I've seen before include "little scruff" , "Scruffy urchin" , "Waif" , "Fuming afar (anag)" , "Poor mite" .)