Plant US drugs policeman is at the outset setting up secretly (9)
I believe the answer is:
narcissus
'plant' is the definition.
The answer is an example.
'us drugs policeman is at the outset setting up secretly' is the wordplay.
'us drugs policeman' becomes 'narc' (narcotics officer).
'at the outset' says to take the initial letters (the outset is the beginning).
The initial letters of 'setting up secretly' is 'sus'.
'narc'+'is'+'sus'='NARCISSUS'
(Other definitions for narcissus that I've seen before include "Flower like daffodil" , "In Greek myth, youth in love with his reflection" , "Beautiful youth; plant" , "Legendary youth who fell in love with his own reflection" , "Greek youth who fell in love with his own reflection" .)