Cathy is at sea in 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 is at sea' is the wordplay.
'is at sea' is an anagram indicator.
'cathy' with letters rearranged gives 'YACHT'.
'in' is the link.
(Other definitions for yacht that I've seen before include "Sailing boat" , "Pleasure craft" , "Type of boat" , "Luxury purchase" , "Light sailing-vessel for sport" .)