A doctor required - about to turn yellow (5)
I believe the answer is:
amber
'turn yellow' is the definition.
'amber' can be an answer for 'yellow' (amber is a kind of yellow). I am not sure about the 'turn' bit.
'a doctor required about to' is the wordplay.
'doctor' becomes 'MB' (Bachelor of Medicine, Medicinae Baccalaureus).
'required' means one lot of letters go next to another.
'about to' becomes 'er' (I can't explain this - if you can you should give a lot more credence to this answer).
'a'+'mb'+'er'='AMBER'
Can you help me to learn more?
(Other definitions for amber that I've seen before include "Middle light" , "Traffic light colour" , "Light giving warning" , "Yellow-brown colour as of bream" , "gemstone" .)