Double top to start, then it's victory! (4)
I believe the answer is:
twin
'double' is the definition.
(thesaurus)
'top to start then it's victory' is the wordplay.
'to start' says to take the initial letters.
'then' says to put letters next to each other.
'it's victory' becomes 'win' (I've seen this before).
The initial 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" .)