Grow old under doctor: its all up to the last letter (5)
I believe the answer is:
omega
'the last letter' is the definition.
(last letter of the Greek alphabet)
'grow old under doctor its all up' is the wordplay.
I cannot quite understand how this works, but
'old' could be 'o' (common abbreviation eg in OE for Old English) and 'o' is present in the answer.
'doctor' could be 'me' and 'me' is located in the answer.
This may be the basis of the clue (or it may be nonsense).
'to' is the link.
Can you help me to learn more?
(Other definitions for omega that I've seen before include "Art of story-telling by body movement" , "Athenian character" , "Last thing" , "Last (Gk. letter)" , "Final Greek letter" .)