Small beer following fish, or gammon (10)
I believe the answer is:
codswallop
'gammon' is the definition.
Both the definition and answer are singular nouns.
Perhaps they are linked in a way I don't understand?
'small beer following fish' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'beer' becomes 'wallop' (I've seen this before).
'following' says to put letters next to each other.
'fish' becomes 'cod' (type of fish).
's'+'wallop'='swallop'
'swallop' after 'cod' is 'CODSWALLOP'.
'or' acts as a link.
Can you help me to learn more?
(Other definitions for codswallop that I've seen before include "waffle?" , "tripe" , "Hogwash" , "[BUNKUM]" , "moonshine" .)