Crustacean is lean crab, minced (8)
I believe the answer is:
barnacle
'crustacean' is the definition.
(barnacle is a kind of crustacean)
'lean crab minced' is the wordplay.
'minced' indicates anagramming the letters.
'lean'+'crab'='leancrab'
'leancrab' with letters rearranged gives 'BARNACLE'.
'is' acts as a link.
(Other definitions for barnacle that I've seen before include "Clinging sort" , "Crustacean that attaches to underwater surfaces" , "type of goose" , "hanger-on" , "'Marine crustacean, attaches to a boat-hull (8)'" .)