Everyone enters fish trade? Nonsense! (10)
I believe the answer is:
codswallop
'nonsense' is the definition.
(I've seen this before)
'everyone enters fish trade?' is the wordplay.
'everyone' becomes 'all' (I've seen this before).
'enters' is an insertion indicator.
'fish' becomes 'cod' (cod is a kind of fish).
'trade?' becomes 'swop' ('swop' can be a synonym of 'trade'**).
'cod'+'swop'='codswop'
'all' going into 'codswop' is 'CODSWALLOP'.
(Other definitions for codswallop that I've seen before include "Nonsense (colloq.)" , "Baloney" , "waffle?" , "Hogwash" , "moonshine" .)