Ban Clare going out to find a crustacean (8)
I believe the answer is:
barnacle
'a crustacean' is the definition.
(I know that Barnacle is a type of crustacean)
'ban clare going out' is the wordplay.
'going out' indicates anagramming the letters (out can mean wrong or inaccurate).
'ban'+'clare'='banclare'
'banclare' is an anagram of 'BARNACLE'.
'to find' is the link.
(Other definitions for barnacle that I've seen before include "Sea creature - lean crab (anag)" , "Crustacean that adheres to ships' bottom" , "'Marine crustacean, attaches to a boat-hull (8)'" , "Be carnal consuming this shellfish" , "One reluctant to leave" .)