Relative intervenes when one becomes old (7)
I believe the answer is:
stepson
'relative' is the definition.
(stepson is a kind of relative)
'intervenes when one becomes old' is the wordplay.
I cannot quite understand how this works, but
'when' could be 'on' (I've seen this in other clues) and 'on' is present in the answer.
The remaining letters 'steps' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for stepson that I've seen before include "No pests (anag)" , "underfoot?" , "boy not sharing blood" , "Child from a spouse's former marriage" , "Spouse's child from previous marriage" .)