In advanced age nervously turning traitor (8)
I believe the answer is:
renegade
'traitor' is the definition.
(I've seen this before)
'in advanced age nervously turning' is the wordplay.
'in' indicates a hidden word.
'turning' says the letters should be written backwards.
'edagener' is hidden in the letters of 'advanced age nervously'.
'edagener' backwards is 'RENEGADE'.
(Other definitions for renegade that I've seen before include "not someone true to the cause" , "Perfidious" , "Deserter, turncoat" , "Betrayer, deserter" , "One who deserts a cause or group" .)