Incredibly rich, Fay's getting crustacean (8)
I believe the answer is:
crayfish
'crustacean' is the definition.
(crayfish is a kind of crustacean)
'incredibly rich fay's getting' is the wordplay.
'incredibly' indicates anagramming the letters.
'getting' is a charade indicator (letters next to each other).
'rich'+'fays'='richfays'
'richfays' with letters rearranged gives 'CRAYFISH'.
(Other definitions for crayfish that I've seen before include "Lobster-like crustacean" , "seafood" , "Lobster-like creature" , "Catch" , "Edible crustacean" .)