Doctor filling in locum roster (7)
I believe the answer is:
oculist
'doctor' is the definition.
(oculist is a kind of doctor)
'filling in locum roster' is the wordplay.
'filling in' says to take the centre (the content or filling of the word).
'roster' becomes 'list' (a roster is a register or list of people).
The central letters of 'locum' are 'ocu'.
'ocu'+'list'='OCULIST'
(Other definitions for oculist that I've seen before include "Specialist in treatment of the eyes" , "Pupils' examiner" , "Old-fashioned eye doctor" , "Visionary healer" , "Person of vision" .)