Well I never form another union (6)
I believe the answer is:
really
This clue is a double definition.
'well i never' is the first definition.
(both interjections expressing surprise)
'form another union' is the second definition.
(re-ally - ally again)
(Other definitions for really that I've seen before include "In actual fact" , "In truth - one may be surprised to say it" , "Genuinely" , "without a doubt" , "Go on" .)