Fish's barrier to work? What nonsense! (10)
I believe the answer is:
codswallop
'nonsense' is the definition.
(I know that nonsense can be written as codswallop)
'fish's barrier to work?' is the wordplay.
'fish's' becomes 'cod's' (type of fish).
'barrier' becomes 'wall' (a wall acts as a barrier).
'to work?' becomes 'op' (abbreviation for opus).
'cods'+'wall'+'op'='CODSWALLOP'
'what' acts as a link.
(Other definitions for codswallop that I've seen before include "Baloney" , "moonshine" , "Nonsense (colloq.)" , "Rubbish!" , "waffle?" .)