Top worker to call teacher (10)
I believe the answer is:
ringmaster
'top worker' is the definition.
(someone working in a circus top)
'call teacher' is the wordplay.
'call' becomes 'ring' (telephone call).
'teacher' becomes 'master' (I've seen this before).
'ring'+'master'='RINGMASTER'
'to' acts as a link.
(Other definitions for ringmaster that I've seen before include "Top top man" , "Circus leader" , "who cracks the whip?" , "Circus controller" , "showman" .)