Proposal held back by aristocratic medical specialist (13)
I believe the answer is:
paediatrician
'medical specialist' is the definition.
(paediatrician is a kind of medical specialist)
'proposal held back by aristocratic' is the wordplay.
I cannot really understand how this works, but
'by' could be 'at' and 'at' is present in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for paediatrician that I've seen before include "Specialist in children and their diseases" , "Child's doctor" , "Specialist in treating children" , "Child doctor" .)