Cathy rocks the boat (5)
I believe the answer is:
yacht
'boat' is the definition.
(I know that yacht is a more specific form of the action boat)
'cathy rocks' is the wordplay.
'rocks' indicates anagramming the letters.
'cathy' anagrammed gives 'YACHT'.
'the' acts as a link.
(Other definitions for yacht that I've seen before include "Kind of boat" , "Luxury purchase" , "Light, fast sailing-vessel" , "Light sailing vessel" , "Small pleasure boat" .)