Migratory fish (6)
I believe the answer is:
salmon
'migratory fish' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for salmon that I've seen before include "Fish (often smoked)" , "Sockeye, for example" , "Parr or grilse" , "'No alms, oddly, for this fish (6)'" , "Oddly, Mal's no fish" .)