Union rioting about cold papal emissary (6)
I believe the answer is:
nuncio
'papal emissary' is the definition.
(I've seen this before)
'union rioting about cold' is the wordplay.
'rioting' indicates an anagram.
'about' indicates putting letters inside.
'cold' becomes 'c' (eg on taps).
'union' is an anagram of 'nunio'.
'nunio' placed around 'c' is 'NUNCIO'.
(Other definitions for nuncio that I've seen before include "Papal ambassador" , "Pope's diplomatic representative" , "Papal emissary" , "Sent by the Pope" , "Papal envoy" .)