Ship’s officer drinking tea he buys (9)
I believe the answer is:
purchaser
'he buys' is the definition.
(I've seen this before)
'ship's officer drinking tea' is the wordplay.
'ship's officer' becomes 'purser' (purser is a kind of ship's officer).
'drinking' indicates putting letters inside.
'tea' becomes 'cha' (cha is a type of tea).
'purser' enclosing 'cha' is 'PURCHASER'.
(Other definitions for purchaser that I've seen before include "One who buys something" , "Buyer" , "he's bought" , "I'll buy" , "customer" .)