Boring teacher struggling with class — headmaster finally intervenes (13)
I believe the answer is:
characterless
'boring' is the definition.
(characterless means boring or uninteresting)
'teacher struggling with class headmaster finally intervenes' is the wordplay.
'struggling with' is an anagram indicator (the letters struggle or wriggle into a new form).
'finally' says to take the final letters.
'intervenes' indicates putting letters inside.
The final letter of 'headmaster' is 'r'.
'teacher' with its letters anagrammed with 'class' makes 'chaacterless'.
'chaacterless' enclosing 'r' is 'CHARACTERLESS'.
(Other definitions for characterless that I've seen before include "Dull" , "Lacking a distinctive nature" , "Lacking distinctive qualities" , "Nondescript" .)