Drink supplied by doctor in the morning (4)
I believe the answer is:
dram
'drink' is the definition.
(small alcoholic drink)
'supplied by doctor in the morning' is the wordplay.
'supplied by' means one lot of letters go next to another.
'doctor' becomes 'dr' (abbreviation).
'in the morning' becomes 'am' (AM on a clock).
'dr' next to 'am' is 'DRAM'.
(Other definitions for dram that I've seen before include "Small drink of spirits" , "Small drink of whisky" , "Short" , "Small drink of scotch" , "Wee drink" .)