The key to what you were once (4)
I believe the answer is:
thee
'what you were once' is the definition.
(I've seen this in another clue)
'the key' is the wordplay.
'key' becomes 'e' (musical key).
'the'+'e'='THEE'
'to' is the link.
(Other definitions for thee that I've seen before include "You in the olden days" , "Poetically you" , "Old objective second person singular" , "You (arch.)" , "'U'" .)