Let's rob sort of crustacean (7)
I believe the answer is:
lobster
'crustacean' is the definition.
(I know that lobster is a type of decapod)
'let's rob sort' is the wordplay.
'sort' indicates anagramming the letters.
'lets'+'rob'='letsrob'
'letsrob' with letters rearranged gives 'LOBSTER'.
'of' is the link.
(Other definitions for lobster that I've seen before include "one for the pot?" , "Bolster up te shellfish" , "It turns bright-red when cooked" , "Edible marine crustacean" , "potted" .)