Investigated no amateur teacher (6)
I believe the answer is:
probed
'investigated' is the definition.
(probing is a kind of investigating)
'no amateur teacher' is the wordplay.
'no amateur' becomes 'pro' (I've seen this in another clue).
'teacher' becomes 'BEd' (holder of a BEd, Bachelor of Education degree).
'pro'+'bed'='PROBED'
(Other definitions for probed that I've seen before include "Did research" , "Investigated closely as dentist does a tooth" , "Examined, dug into" , "Searched" , "Investigating closely" .)