Enduring wrong to arrest president, ultimately (8)
I believe the answer is:
immortal
'enduring' is the definition.
(I've seen this before)
'wrong to arrest president ultimately' is the wordplay.
'wrong' becomes 'immoral' (similar in meaning).
'to arrest' is an insertion indicator (inserted letters are arrested or held).
'president ultimately' becomes 't' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'immoral' placed around 't' is 'IMMORTAL'.
Can you help me to learn more?
(Other definitions for immortal that I've seen before include "Moral Tim (anag.)" , "Never dying" , "Undying" , "Living forever" , "Deathless" .)