Accepted member wearing crimson is backed (7)
I believe the answer is:
insider
'accepted member' is the definition.
(I've seen this in another clue)
'wearing crimson is backed' is the wordplay.
'wearing' becomes 'in' (in certain clothing).
'crimson' becomes 'red' (synonyms).
'backed' is a reversal indicator.
'red'+'is'='redis'
'redis' back-to-front is 'sider'.
'in'+'sider'='INSIDER'
(Other definitions for insider that I've seen before include "One privy to info by being within organisation" , "Particularly advantaged person" , "Member of a specific group" , "He's in the circle, knows the right people" , "one's not allowed to trade" .)