Crustacean Rob lets out (7)
I believe the answer is:
lobster
'crustacean' is the definition.
(I know that lobster is a type of decapod)
'rob lets out' is the wordplay.
'out' is an anagram indicator (out can mean wrong or inaccurate).
'rob'+'lets'='roblets'
'roblets' with letters rearranged gives 'LOBSTER'.
(Other definitions for lobster that I've seen before include "Pincered crustacean, a delicacy" , "food?" , "Crustacean with pincer-like claws" , "one to loft a > shellfish?" , "One showing claws" .)