Doctor, in the morning, has a drink (4)
I believe the answer is:
dram
'a drink' is the definition.
(small alcoholic drink)
'doctor in the morning' is the wordplay.
'doctor' becomes 'dr' (abbreviation).
'in the morning' becomes 'am' (AM on a clock).
'dr'+'am'='DRAM'
'has' acts as a link.
(Other definitions for dram that I've seen before include "Small drink (in Scotland)" , "Wee drink" , "little weight" , "Small drink of scotch" , "Small drink of alcohol" .)