Everyone enters fish trade? Nonsense! (10)
I believe the answer is:
Here is my best explanation:
'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'**).
'all' going into 'codswop' is 'CODSWALLOP'.
(Other definitions for codswallop that I've seen before include "Rubbish!", "Nonsense (colloq.)", "Baloney", "waffle?", "moonshine", "Moonshine", "Hogwash", "nonsense", "tripe" and "[BUNKUM]".)