Extra, and almost certain, advantage (7)
I believe the answer is:
surplus
'extra' is the definition.
(I know that extra can be written as surplus)
'almost certain advantage' is the wordplay.
'almost' means to remove the last letter (most of the word but not all of it).
'certain' becomes 'sure' (synonyms).
'advantage' becomes 'plus' (a plus is an advantage or benefit).
'sure' with its final letter taken off is 'sur'.
'sur'+'plus'='SURPLUS'
'and' is the link.
(Other definitions for surplus that I've seen before include "(An) oversupply" , "balance" , "a bit over the top" , "superfluity" , "Unneeded extra" .)