Fill an opening for a locum (7)
I believe the answer is:
stopgap
'fill' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
'an opening for a locum' is the wordplay.
'an opening' becomes 'gap' (gap is a kind of opening).
'for' is a charade indicator (letters next to each other) (I've seen this in other clues).
'a locum' becomes 'stop' (I am not sure about this - if you are sure you should believe this answer much more).
'gap' put after 'stop' is 'STOPGAP'.
Can you help me to learn more?
(Other definitions for stopgap that I've seen before include "Temporary fix" , "Makeshift" , "Provisional" , "Improvisation" , "Interim" .)