First motion of club supports extension (9)

I believe the answer is:

backswing

'first motion of club' is the definition.
(as in a golf club)

'supports extension' is the wordplay.
'supports' becomes 'backs' ('back' can be a synonym of 'support').
'extension' becomes 'wing' ('wing' can be a synonym of 'extension').
'backs'+'wing'='BACKSWING'

I've seen this clue in The Guardian.

Want a hint initially instead of a full solution? Install my app