Greeting socialist who's engaged (5)
I believe the answer is:
hired
'engaged' is the definition.
('hire' can be a synonym of 'engage')
'greeting socialist' is the wordplay.
'greeting' becomes 'hi' (hi is a kind of greeting).
'socialist' becomes 'red' (socialism is often associated with the colour red).
'hi'+'red'='HIRED'
'who's' is the link.
(Other definitions for hired that I've seen before include "Paid to work" , "Employed or engaged" , "Rented out" , "like a Merc" , "Engaged for wages" .)