Attention required in class — might one be leaning on desk? (7)
I believe the answer is:
forearm
'might one be leaning on desk?' is the definition.
'attention required in class' is the wordplay.
'attention' becomes 'ear' (ear is a kind of attention).
'required in' means one lot of letters goes inside another.
'class' becomes 'form' (form class is a kind of class).
'ear' placed into 'form' is 'FOREARM'.
(Other definitions for forearm that I've seen before include "Before battle, take up weapon" , "where is the radius?" , "Attention" , "what's near to hand?" , "Member's part" .)