An old one could be an ash blonde (5)
I believe the answer is:
flame
'be an ash blonde' is the definition.
Although both the answer and definition are singular nouns, I don't understand how one could define the other.
'an old one' is the wordplay.
'an old' becomes 'fla' (I can't explain this - if you can you should believe this answer much more).
'one' becomes 'me' (a setter might use 'one' to humourously mean 'me').
'fla'+'me'='FLAME'
'could' is the link.
Can you help me to learn more?
(Other definitions for flame that I've seen before include "Burner -- passion" , "Glowing part of fire" , "abusive comment" , "Orange-red colour" , "Evidence of fire" .)