Messenger joining a new set (5)
I believe the answer is:
angel
'messenger' is the definition.
(heavenly messenger)
'joining a new set' is the wordplay.
'joining' 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' put next to 'n' is 'an'.
'an'+'gel'='ANGEL'
(Other definitions for angel that I've seen before include "Spiritual being - financial backer" , "Investor in a theatrical production" , "somewhere in Islington" , "Person of beauty, purity or kindness?" , "One backing theatre production" .)