Union to scale down working during month (9)
I believe the answer is:
matrimony
'union' is the definition.
(synonyms)
'scale down working during month' is the wordplay.
'scale down' becomes 'trim' (both can mean to reduce).
'working' becomes 'on' (on can mean working or activated).
'during' indicates putting letters inside.
'month' becomes 'may' (May is an example).
'trim'+'on'='trimon'
'trimon' put into 'may' is 'MATRIMONY'.
'to' is the link.
(Other definitions for matrimony that I've seen before include "State of being married" , "The state of wedlock" , "Nuptials" , "wedding ceremony" .)