Double top to start, then it's victory! (4)
I believe the answer is:
twin
'double' is the definition.
(twin can mean someone's double)
'top to start then it's victory' is the wordplay.
'to start' indicates taking the first letters.
'then it's' says to put letters next to each other.
'victory' becomes 'win' (a win is a victory).
The first letter of 'top' is 't'.
't'+'win'='TWIN'
(Other definitions for twin that I've seen before include "(Town) paired with another" , "Dead ringer" , "One of Gemini" , "Very much like" , "Balearic island" .)