Just tell one student to hide answer (9)
I believe the answer is:
impartial
'just' is the definition.
(I've seen this before)
'tell one student to hide answer' is the wordplay.
'tell' becomes 'impart' (imparting is a kind of telling**).
'one' becomes 'i' (Roman numeral).
'student' becomes 'l'.
'to hide' is an insertion indicator.
'answer' becomes 'a' (abbreviation eg in Q and A).
'i'+'l'='il'
'il' going around 'a' is 'ial'.
'impart'+'ial'='IMPARTIAL'
(Other definitions for impartial that I've seen before include "Fair, unprejudiced" , "Treating all rivals equally" , "Disinterested" , "not taking sides" , "Objective" .)