Lag, one put inside when students attend school (7)
I believe the answer is:
interim
'lag' is the definition.
(synonyms)
'one put inside when students attend school' is the wordplay.
I cannot really see how this works, but
'one' could be 'er' (I've seen this in other clues) and 'er' is present in the answer.
'inside' could be 'in' and 'in' is found in the answer.
The remaining letters 'tim' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
(Other definitions for interim that I've seen before include "acting" , "Meanwhile" , ", I'm put in grave situation initially" , "Temporary - provisional" , "Meantime - provisional" .)