Alert, showing leg before the dance (2,3,4)
I believe the answer is:
on the ball
'alert' is the definition.
(on the ball can mean alert or paying attention)
'leg before the dance' is the wordplay.
'leg' becomes 'on' (cricket term, fielding positions).
'before' means one lot of letters go next to another.
'dance' becomes 'ball' (ball is a kind of dance).
'on'+'the'+'ball'='ON THE BALL'
'showing' acts as a link.
(Other definitions for on the ball that I've seen before include "Alert and focused as soccer player should be" , "Savvy" , "Properly alert" , "Intelligent" , "Alert and practical - during soccer match?" .)