Being old, one gets pain (8)
I believe the answer is:
existing
'being' is the definition.
The definition and answer can be both to do with fundamental existence as well as being present participle verbs.
Perhaps you can see a link between them that I can't see?
'old one gets pain' is the wordplay.
'old' becomes 'ex'.
'one' becomes 'I' (Roman numeral).
'gets' is a charade indicator (letters next to each other).
'pain' becomes 'sting' (sting is a kind of pain).
'ex'+'i'+'sting'='EXISTING'
Can you help me to learn more?
(Other definitions for existing that I've seen before include "To be found in the real world" , "Extant, alive" , "Surviving" , "Currently living" , "Having place as part of reality" .)