Regularly trusty — our teacher (5)
I believe the answer is:
tutor
'teacher' is the definition.
(tutor is a kind of teacher)
'regularly trusty our' is the wordplay.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
'trusty'+'our'='trustyour'
The alternate letters of 'trustyour' are 'TUTOR'.
(Other definitions for tutor that I've seen before include "Private or university teacher" , "Edify" , "Personal teacher" , "Master" , "class leader" .)