Marine crustacean, a sort of lean crab (8)
I believe the answer is:
barnacle
'marine crustacean' is the definition.
(I've seen this in another clue)
'a sort of lean crab' is the wordplay.
'a sort of' is an anagram indicator.
'lean'+'crab'='leancrab'
'leancrab' is an anagram of 'BARNACLE'.
(Other definitions for barnacle that I've seen before include "Marine crustacean - European goose" , "Shellfish that sticks to things" , "One that sticks" , "type of goose" , "Marine crustacean (attaching to rocks, ships etc.)" .)