In number three cabin? (5)
I believe the answer is:
berth
'cabin?' is the definition.
(thesaurus)
'in number three' is the wordplay.
'in' indicates the answer is hidden within the clue.
'BERTH' is hidden within 'number three'.
(Other definitions for berth that I've seen before include "Place to lie down on train or ship" , "Accommodation on a ship" , "Room on a ship" , "Bunk; mooring place" , "Assigned place" .)