Crossword Genius

Substitute in gym class over there does short stint outside (4,2)

Ross

I believe the answer is:

step in

I'm a little stuck... Click here to teach me more about this clue!

'substitute' is the definition.
(I know that substitute can be written as step in)

'in gym class over there does short stint outside' is the wordplay.
I cannot really see how this works, but
'in' is present in the answer.
The remaining letters 'step' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...

Can you help me to learn more?

(Other definitions for step in that I've seen before include "Intervene dramatically, or at a senior level" , "enter" , "Shoe with no fastenings" , "Interrupt" , "Type of garment" .)

I've seen this clue in The Irish Times.
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