Securing first hit after fish nonsense (10)
I believe the answer is:
codswallop
'nonsense' is the definition.
(I've seen this before)
'securing first hit after fish' is the wordplay.
'first' suggests taking the first letters.
'hit' becomes 'wallop' (walloping is a kind of hitting).
'after' is a charade indicator (letters next to each other).
'fish' becomes 'cod' (type of fish).
The first letter of 'securing' is 's'.
's'+'wallop'='swallop'
'swallop' after 'cod' is 'CODSWALLOP'.
(Other definitions for codswallop that I've seen before include "Hogwash" , "Baloney" , "Nonsense (colloq.)" , "[BUNKUM]" , "tripe" .)