Completely finished, the fish, we hear — everywhere! (3,4,3,5)
I believe the answer is:
all over the place
'everywhere' is the definition.
(I've seen this in another clue)
'completely finished the fish we hear' is the wordplay.
'completely finished' becomes 'all over' (I've seen this before).
'fish' becomes 'plaice' (plaice is a kind of fish).
'we hear' indicates a 'sounds like' (homophone) clue.
'plaice' is a homophone of 'place'.
'allover'+'the'+'place'='ALL OVER THE PLACE'
(Other definitions for all over the place that I've seen before include "mess" , "not orderly!" , "Confused" , "everywhere" , "in disarray" .)