Greeting socialist who's taken on (5)
I believe the answer is:
hired
'taken on' is the definition.
(I've seen this in another clue)
'greeting socialist' is the wordplay.
'greeting' becomes 'hi' (hi is a kind of greeting).
'socialist' becomes 'red' (red can informally mean a socialist).
'hi'+'red'='HIRED'
'who's' acts as a link.
(Other definitions for hired that I've seen before include "Chartered" , "Taken into service" , "Employed or engaged" , "Rented out" , "Employed for wages" .)