Top young actor casually hired twentieth 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)
'top young actor casually hired twentieth' is the wordplay.
'top' says to take the initial letters (in a down clue, the first letter is top).
The initial letters of 'young actor casually hired twentieth' is 'YACHT'.
(Other definitions for yacht that I've seen before include "Pleasure craft" , "Small pleasure boat" , "Sailing boat" , "Fast sailing vessel" , "Type of boat" .)