Club member mostly to avoid upset (6)
I believe the answer is:
cudgel
'club' is the definition.
(I know that club can be written as cudgel)
'member mostly to avoid upset' is the wordplay.
'member' becomes 'leg' (member can mean a limb).
'mostly' means to remove the last letter.
'to avoid' becomes 'duck' (duck can mean to avoid).
'upset' says the letters should be written in reverse.
'duck' with its final letter removed is 'duc'.
'leg'+'duc'='legduc'
'legduc' in reverse letter order is 'CUDGEL'.
(Other definitions for cudgel that I've seen before include "Heavy staff, club" , "Short thick stick used as weapon" , "A ruffian's club" , "Truncheon-like weapon" , "Bludgeon" .)