Cool doctor under retro dad look (6)
I believe the answer is:
aplomb
'cool' is the definition.
(synonyms)
'doctor under retro dad look' is the wordplay.
'doctor' becomes 'mb' (Bachelor of Medicine, Medicinae Baccalaureus).
'under' means one lot of letters go next to another (below, in a down clue).
'retro' shows that the letters should be reversed in order (retro is Latin for 'backwards').
'dad' becomes 'pa' (pa can mean someone's father).
'look' becomes 'lo' (archaic exclamation meaning 'look!').
'pa' reversed gives 'ap'.
'ap'+'lo'='aplo'
'mb' put after 'aplo' is 'APLOMB'.
(Other definitions for aplomb that I've seen before include "Self-satisfied dignity, complacency" , "Air of self-satisfied confidence" , "Cool self-confidence" , "Self-satisfied dignity and confidence" , "Pompous dignity or poise" .)