Doctor back to nurse one glutton (3)
I believe the answer is:
pig
'glutton' is the definition.
(I've seen this in another clue)
'doctor back to nurse one' is the wordplay.
'doctor' becomes 'gp' (general practitioner).
'back' shows that the letters should be reversed in order.
'to nurse' means one lot of letters goes inside another.
'one' becomes 'i' (Roman numeral).
'gp' written backwards gives 'pg'.
'pg' enclosing 'i' is 'PIG'.
(Other definitions for pig that I've seen before include "Glutton" , "Boar or sow" , "Greedy person (colloq.)" , "Farmyard animal" , "Domestic swine" .)