Substitute in gym class over there does short stint outside (4,2)
I believe the answer is:
step in
'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" .)