Downfall when union oddly stops helping (9)
I believe the answer is:
ruination
'downfall' is the definition.
(thesaurus)
'union oddly stops helping' is the wordplay.
'oddly' indicates alternate letters.
'stops' means one lot of letters goes inside another.
'helping' becomes 'ration' (I've seen this before).
The alternate letters of 'union' are 'uin'.
'uin' going into 'ration' is 'RUINATION'.
'when' acts as a link.
(Other definitions for ruination that I've seen before include "In financial disaster" , "Complete destruction or collapse" , "State of loss" , "Complete downfall" , "bankruptcy" .)