Everyone enters fish trade? Nonsense! (10)
I believe the answer is:
Here is my best explanation:
'nonsense' is the definition.
'everyone enters fish trade?' is the subsidiary indication.
'everyone' becomes 'all' (**).
'enters' indicates putting letters inside.
'fish' becomes 'cod' (cod is a kind of fish).
'trade?' becomes 'swop' ('swop' can be a synonym of 'trade').
'all' inserted within 'codswop' is 'codswallop'.
(Other definitions for codswallop that I've seen before include "Nonsense (colloq.)", "Baloney" and "Hogwash".)