A posh girl beginning to sing articulated 'Forever Young' (7)
I believe the answer is:
ageless
'forever young' is the definition.
(I've seen this before)
'a posh girl beginning to' is the wordplay.
'posh girl' becomes 'gel' (I've seen this in other clues).
'beginning to' becomes 'ess' (I can't explain this - if you can you should believe this answer much more).
'a'+'gel'+'ess'='AGELESS'
'sing articulated' acts as a link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for ageless that I've seen before include "Not growing older" , "Not appearing to grow older" , "Long-lasting -- sea legs (anag)" , "Staying young" , "Sea-legs (anag.)" .)