Extremely old man accepting fate at last (4)
I believe the answer is:
dead
'extremely' is the definition.
(as in a phrase like 'dead easy')
'old man accepting fate at last' is the wordplay.
'old man' becomes 'dad' (synonyms).
'accepting' indicates putting letters inside.
'at last' says to take the final letters.
The final letter of 'fate' is 'e'.
'dad' enclosing 'e' is 'DEAD'.
(Other definitions for dead that I've seen before include "directly" , "Pushing up the daisies" , "Passed on" , "dull" , "Not functioning; numb" .)