Crustacean Rob lets loose (7)
I believe the answer is:
lobster
'crustacean' is the definition.
(lobster is a kind of crustacean)
'rob lets loose' is the wordplay.
'loose' is an anagram indicator (letters are loosened from their original positions).
'rob'+'lets'='roblets'
'roblets' anagrammed gives 'LOBSTER'.
(Other definitions for lobster that I've seen before include "Large marine decapod" , "Bolster (anag.)" , "Lets bro have the shellfish" , "Marine creature" , "Tommy no longer" .)