Doctor has morning drink (4)
I believe the answer is:
dram
'drink' is the definition.
(small alcoholic drink)
'doctor has morning' is the wordplay.
'doctor' becomes 'dr' (abbreviation).
'has' means one lot of letters go next to another.
'morning' becomes 'am' (as in 12:00 AM).
'dr'+'am'='DRAM'
(Other definitions for dram that I've seen before include "Pick-me-up" , "A wee spirit" , "Measure of spirits" , "Small drink of alcohol" , "Measure of Scotch" .)