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Brief one on how to be a substitute? (4,2)

Ross

I believe the answer is:

fill in

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'a substitute?' is the definition.
(I've seen this before)

I cannot understand how the remainder of the clue works.

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(Other definitions for fill in that I've seen before include "Brief" , "Deputise - complete form" , "'Act as temporary substitute (4,2)'" , "Occupy (time)" , "point between courses" .)

I've seen this clue in the Evening Standard.
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