Trained first of teaching assistants, yours truly (4)
I believe the answer is:
tame
'trained' is the definition.
(thesaurus)
'first of teaching assistants yours truly' is the wordplay.
'first of' indicates taking the first letters.
'yours truly' becomes 'me' (the person who set this crossword).
The first letters of 'teaching assistants' is 'ta'.
'ta'+'me'='TAME'
(Other definitions for tame that I've seen before include "Submissive -- pedestrian" , "Not exciting; cooperative" , "Domesticated kind of meat" , "Domisticated with meat" , "Domesticated, not wild" .)