Seller's against completion before October vacation (6)
I believe the answer is:
vendor
'seller's' is the definition.
(synonyms)
'against completion before october vacation' is the wordplay.
'against' becomes 'v' (abbreviation for 'versus').
'completion' becomes 'end' (I've seen this before).
'before' means one lot of letters go next to another.
'october vacation' becomes 'or' (I can't explain this - if you can you should believe this answer much more).
'v'+'end'+'or'='VENDOR'
Can you help me to learn more?
(Other definitions for vendor that I've seen before include "Person selling" , "Purveyor" , "Person who sells goods" , "One selling house" , "who does business?" .)