Being old is no good, receding relations admitted (8)
I believe the answer is:
existing
'being' is the definition.
The answer and definition can be both to do with fundamental existence as well as being present participle verbs.
Perhaps you can see an association between them that I can't see?
'old is no good receding relations admitted' is the wordplay.
'old' becomes 'ex' (prefix meaning former or onetime).
'no good' becomes 'ng' (abbreviation).
'receding' is a reversal indicator.
'relations' becomes 'it' (both can mean sex).
'admitted' is an insertion indicator (inserted letters are admitted in).
'it' reversed gives 'ti'.
'is'+'ng'='isng'
'isng' going around 'ti' is 'isting'.
'ex'+'isting'='EXISTING'
Can you help me to learn more?
(Other definitions for existing that I've seen before include "Surviving" , "Extant, alive" , "Having place as part of reality" , "Being present in the world" , "To be found in the real world" .)