Phone teacher who may send in the clowns? (10)
I believe the answer is:
ringmaster
'phone teacher who may send in the clowns?' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
'phone teacher' is the wordplay.
'phone' becomes 'ring' (to ring on the phone).
'teacher' becomes 'master' (I've seen this before).
'ring'+'master'='RINGMASTER'
Can you help me to learn more?
(Other definitions for ringmaster that I've seen before include "circus performer" , "Top top man" , "who cracks the whip?" , "Circus compere" , "Circus controller" .)