A medical man goes on about turning yellow (5)
I believe the answer is:
amber
'yellow' is the definition.
(amber is a kind of yellow)
'a medical man goes on about turning' is the wordplay.
'medical man' becomes 'MB' (someone with a Bachelor of Medicine degree).
'goes on' means one lot of letters go next to another.
'about' becomes 're' (regarding).
'turning' shows that the letters should be reversed in order.
're' in reverse letter order is 'er'.
'a'+'mb'+'er'='AMBER'
(Other definitions for amber that I've seen before include "A yellowy resin" , "Traffic-light colour" , "Light giving warning" , "Yellow-orange" , "Medium to dark brownish-yellow colour" .)