Greatly affect unionist and await decision (5)
I believe the answer is:
upend
'greatly affect' is the definition.
(I've seen this before)
'unionist and await decision' is the wordplay.
'unionist' becomes 'u' (abbreviation as in Democratic Unionist Party).
'and' means one lot of letters go next to another.
'await decision' becomes 'pend' (I've seen this in another clue).
'u'+'pend'='UPEND'
(Other definitions for upend that I've seen before include "change things drastically" , "Set on side, turn over" , "Make a turnover" , "Set (something) vertical" , "Affect drastically" .)