Being old makes this clue extremely commendable (9)
I believe the answer is:
existence
'being' is the definition.
(I know that being can be written as existence)
'old makes this clue extremely commendable' is the wordplay.
I cannot quite understand how this works, but
'old' could be 'ex' (prefix meaning former or onetime) and 'ex' is found within the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for existence that I've seen before include "State of being" , "A way of living" , "Way of life" , "survival" , "The business of being" .)