Thwarting fellow to lose edge and standing (11)
I believe the answer is:
frustrating
'thwarting' is the definition.
(I know that thwarting can be written as frustrating)
'fellow to lose edge and standing' is the wordplay.
'fellow' becomes 'F' (abbreviation - of a society etc.).
'to lose edge' becomes 'rust' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'and' is a charade indicator (letters next to each other).
'standing' becomes 'rating' (rating is a kind of standing).
'f'+'rust'+'rating'='FRUSTRATING'
Can you help me to learn more?
(Another definition for frustrating that I've seen is " Causing discouragement".)