Sponsor a northern set (5)
I believe the answer is:
angel
'sponsor' is the definition.
(angel is a kind of sponsor)
'a northern set' is the wordplay.
'northern' becomes 'n' (abbreviation e.g. N. Ireland).
'set' becomes 'gel' (both can mean to harden).
'a'+'n'+'gel'='ANGEL'
(Other definitions for angel that I've seen before include "Sponsor" , "Guardian spirit; theatre investor" , "Winged messenger" , "Word linking cake and fish" , "one might be recording" .)