Helping to suppress unions oddly creates havoc (9)
I believe the answer is:
ruination
'havoc' is the definition.
(thesaurus)
'helping to suppress unions oddly' is the wordplay.
'helping' becomes 'ration' (a ration is a serving or helping).
'to suppress' is an insertion indicator.
'oddly' means one should take alternating letters (letters in the odd-numbered positions are used).
The alternating letters of 'unions' are 'uin'.
'ration' placed around 'uin' is 'RUINATION'.
'creates' is the link.
(Other definitions for ruination that I've seen before include "Complete downfall" , "bankruptcy" , "Utter destruction" , "total disaster" , "End" .)