Serious from outset, deserve to be ahead (7)
I believe the answer is:
earnest
'serious' is the definition.
(similar in meaning)
'outset deserve to be ahead' is the wordplay.
'outset' becomes 'est' (I am not sure about this - if you are sure you should believe this answer much more).
'deserve' becomes 'earn' (I've seen this before).
'to be ahead' says to put letters next to each other.
'est' after 'earn' is 'EARNEST'.
'from' is the link.
(Other definitions for earnest that I've seen before include "Serious - down payment" , "guarantee" , "Firm" , "promise" , "Serious - advance" .)