Crustacean head of classy pub rejected (4)
I believe the answer is:
crab
'crustacean' is the definition.
(I know that crab is a type of decapod)
'head of classy pub rejected' is the wordplay.
'head of' says to take the initial letters.
'pub' becomes 'bar' (both are drinking establishments).
'rejected' says the letters should be written in reverse.
The first letter of 'classy' is 'c'.
'bar' written backwards gives 'rab'.
'c'+'rab'='CRAB'
(Other definitions for crab that I've seen before include "Seafood (that can be caught by a rower?)" , "winch" , "Scuttling crustacean" , "Fiddler, perhaps" , "A crustacean; type of apple" .)