Extra doctor? That's a relief (5)

I believe the answer is:

locum

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This might be a double definition.

'extra doctor?' is the first definition.
'locum' can be an answer for 'doctor?' (I've seen this before). I am unsure of the 'extra' bit.

'relief' is the second definition.
(I know that locum is a type of relief)

'that's a' is the link.

Can you help me to learn more?

(Other definitions for locum that I've seen before include "He fills in" , "Stand-in medic" , "Temporary replacement doctor" , "temporary occupant" , "One stands in" .)

I've seen this clue in The Guardian.

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