Mimic law enforcer unknown on twin-hulled vessel (7)
I believe the answer is:
copycat
'mimic' is the definition.
(I've seen this before)
'law enforcer unknown on twin-hulled vessel' is the wordplay.
'law enforcer' becomes 'cop' (I've seen this in another clue).
'unknown' becomes 'y' (algebra).
'on' says to put letters next to each other.
'twin-hulled vessel' becomes 'cat' (short for catamaran, type of ship).
'cop'+'y'+'cat'='COPYCAT'
(Other definitions for copycat that I've seen before include "Person who imitates another in feline way" , "One who imitates another" , "Slavish imitator" , "Some murders are" , "Imitator - imitative" .)