Special batter added to fish balls (10)
I believe the answer is:
codswallop
'balls' is the definition.
(both can mean nonsense)
'special batter added to fish' is the wordplay.
'special' becomes 'S' (common abbreviation as in Special Air Service).
'batter' becomes 'wallop' (both can mean to hit hard).
'added to' is a charade indicator (letters next to each other).
'fish' becomes 'cod' (type of fish).
's'+'wallop'='swallop'
'swallop' after 'cod' is 'CODSWALLOP'.
(Other definitions for codswallop that I've seen before include "moonshine" , "tripe" , "Baloney" , "[BUNKUM]" , "Rubbish!" .)