One being a substitute in practice? (5)

I believe the answer is:

locum

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'substitute in practice?' is the definition.
'locum' can be an answer for 'substitute' (locum is a kind of substitute). I am not sure about the 'in practice?' bit.

I don't understand the remainder of the clue.

Can you help me to learn more?

(Other definitions for locum that I've seen before include "One standing in" , "one won't be in position for ever!" , "Deputising doctor" , "Temporary replacement doctor" , "Stand-in physician" .)

I've seen this clue in The Telegraph and The Sun.

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