Lean crab moving around marine crustacean (8)
I believe the answer is:
barnacle
'marine crustacean' is the definition.
(I've seen this in another clue)
'lean crab moving' is the wordplay.
'moving' indicates an anagram (letters are moved around).
'lean'+'crab'='leancrab'
'leancrab' anagrammed gives 'BARNACLE'.
'around' is the link.
(Other definitions for barnacle that I've seen before include "'Marine crustacean, attaches to a boat-hull (8)'" , "Hull resident (one who clings on)?" , "A shellfish that sticks to objects under water" , "One reluctant to leave" , "type of goose" .)