Messenger joins a new set (5)
I believe the answer is:
angel
'messenger' is the definition.
(heavenly messenger)
'joins a new set' is the wordplay.
'joins' says to put letters next to each other.
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'set' becomes 'gel' (both can mean to harden).
'a' next to 'n' is 'an'.
'an'+'gel'='ANGEL'
(Other definitions for angel that I've seen before include "Very helpful person" , "Christmas tree decoration" , "darling!" , "Heavenly messenger" , "Investor in a theatrical production" .)