Messenger joins a northern set (5)
I believe the answer is:
angel
'messenger' is the definition.
(heavenly messenger)
'joins a northern set' is the wordplay.
'joins' says to put letters next to each other.
'northern' becomes 'n' (abbreviation e.g. N. Ireland).
'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 "Cloud resident?" , "Theatre supporter" , "Heavenly body" , "Investor in a theatrical production" , "Messenger < is a nurse" .)