Doctor discovered locum left first (7)
I believe the answer is:
oculist
'doctor' is the definition.
(oculist is a kind of doctor)
'discovered locum left first' is the wordplay.
'discovered' says to take the centre (dis-covered - outer letters removed).
'left' becomes 'l' (common abbreviation).
'first' becomes 'ist' (ist resembles 1st).
The middle of 'locum' is 'ocu'.
'ocu'+'l'+'ist'='OCULIST'
(Other definitions for oculist that I've seen before include "Old-fashioned eye doctor" , "Pupils' examiner" , "Man of vision" , "Specialist in treatment of the eyes" , "Visionary healer" .)