Willing girlfriend coming to game — it's a start (5,6,2)
I believe the answer is:
ready steady go
'a start' is the definition.
(phrase used to start a race)
'willing girlfriend coming to game' is the wordplay.
'willing' becomes 'ready' (associated in meaning).
'girlfriend' becomes 'steady' (steady can mean a person's partner).
'coming to' is a charade indicator (letters next to each other).
'game' becomes 'go game' (go game is a kind of game).
'ready'+'steady'+'go'='READY STEADY GO'
'it's' is the link.
(Other definitions for ready steady go that I've seen before include "Words used to start a race" , "that starts competition" , "Race starter's words" , "As may be said by race starter" , "order for competitors" .)