First 6 of 10 could become greatest in years (6)
I believe the answer is:
oldest
'first' is the definition.
('oldest' can be similar in meaning to 'first')
I don't understand how the rest of the clue works.
(Other definitions for oldest that I've seen before include "Most ancient (O, not E)" , "First in time" , "Least recent" , "Let's do! (anag.)" , "Most aged" .)