Doctor one patient, initially, with a transfusion (4)
I believe the answer is:
drip
'a transfusion' is the definition.
(thesaurus)
'doctor one patient initially' is the wordplay.
'doctor' becomes 'dr' ('Dr' can be a synonym of 'doctor').
'one' becomes 'i' (Roman numeral**).
'initially' suggests taking the first letters.
The initial letter of 'patient' is 'p'.
'dr'+'i'+'p'='DRIP'
'with' acts as a link.
(Other definitions for drip that I've seen before include "Weak and pathetic person" , "Hospital device; weak person" , "As a tear appears" , "Sissy" , "Fall as from leaky tap" .)