Approval teacher's withdrawn (3)
I believe the answer is:
nod
'approval' is the definition.
(thesaurus)
'teacher's withdrawn' is the wordplay.
'teacher' becomes 'don' (don is a kind of teacher).
'withdrawn' is a reversal indicator.
'don' in reverse letter order is 'NOD'.
(Other definitions for nod that I've seen before include "sleepy land" , "Silent assent" , "Green light" , "Be agreeably" , "sign of approval" .)