One having uncapped tranquilliser (5)
I believe the answer is:
owner
'one having' is the definition.
(I've seen this before)
'uncapped tranquilliser' is the wordplay.
'uncapped' means to remove the first letter.
'tranquilliser' becomes 'downer' (I've seen this before).
'downer' with its first letter taken away is 'OWNER'.
(Other definitions for owner that I've seen before include "One who has something" , "One who possesses" , "The person in legal possession" , "Possessor, proprietor" , "landlord" .)