Start game unconventionally, making attempt to gain advantage (9)
I believe the answer is:
stratagem
'attempt to gain advantage' is the definition.
(a stratagem is a scheme to beat an opponent)
'start game unconventionally' is the wordplay.
'unconventionally' indicates anagramming the letters.
'start'+'game'='startgame'
'startgame' is an anagram of 'STRATAGEM'.
'making' is the link.
(Other definitions for stratagem that I've seen before include "Plan to deceive enemy" , "could be a trick" , "A cunning plan or scheme" , "Tactic to gain an advantage" , "way to outwit opponent" .)