Don't allow a GP going off to get a locum (7)

I believe the answer is:

stopgap

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'a locum' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.

'don't allow a gp going off to get' is the wordplay.
'don't allow' becomes 'stop' (I've seen this before).
'going off' indicates anagramming the letters (letters go or move around).
'to get' indicates putting letters inside.
'a'+'gp'='agp'
'agp' is an anagram of 'gap'.
'stop' placed inside 'gap' is 'STOPGAP'.

Can you help me to learn more?

(Other definitions for stopgap that I've seen before include "caretaker" , "Temporary fix" , "Makeshift" , "Provisional" , "A temporary or impromptu substitute" .)

I've seen this clue in the Evening Standard.

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