A number finish with an alternative seller (6)
I believe the answer is:
vendor
'seller' is the definition.
(I know that seller can be written as vendor)
'a number finish with an alternative' is the wordplay.
I cannot really see how this works, but
'number' could be 'v' (V is an example) and 'v' is located in the answer.
'finish' could be 'end' (synonyms) and 'end' is present in the answer.
'alternative' could be 'or' and 'or' is located in the answer.
No letters remain.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for vendor that I've seen before include "One selling house" , "Salesperson" , "Purveyor" , "Seller, trader (O, not E)" , "Person who sells goods" .)