Club could occasionally upset member (6)
I believe the answer is:
cudgel
'club' is the definition.
(cudgel is a kind of club)
'could occasionally upset member' is the wordplay.
'occasionally' indicates alternate letters.
'upset' is a reversal indicator.
'member' becomes 'leg' (leg is a kind of member).
The alternating letters of 'could' are 'cud'.
'leg' written backwards gives 'gel'.
'cud'+'gel'='CUDGEL'
(Other definitions for cudgel that I've seen before include "Wooden weapon" , "Stout stick; hit with this" , "Bludgeon" , "Brutish club" , "Club used for hitting people" .)