Ancient part in Armageddon (4)
I believe the answer is:
aged
'ancient' is the definition.
(I've seen this in another clue)
'part in armageddon' is the wordplay.
'part in' indicates the answer is hidden within the clue.
'AGED' is hidden in the letters of 'armageddon'.
(Other definitions for aged that I've seen before include "Senior" , "Grew old" , "Matured like wine in wood" , "Ripened, matured" , "Advanced in years" .)