Like volunteers, give assistance after UN pressure (6)
I believe the answer is:
unpaid
'like volunteers' is the definition.
'assistance after un pressure' is the wordplay.
'assistance' becomes 'aid' ('aid' can be a synonym of 'assistance').
'after' means one lot of letters go next to another.
'pressure' becomes 'p' (abbreviation).
'un'+'p'='unp'
'aid' put after 'unp' is 'UNPAID'.
'give' acts as a link.
(Other definitions for unpaid that I've seen before include "Outstanding, owing" , "Overdue" , "Still owing" , "Without wages" , "Not give wages" .)