Vexation, on dentist failing to protect canines, primarily (10)
I believe the answer is:
discontent
'vexation' is the definition.
(thesaurus)
'on dentist failing to protect canines primarily' is the wordplay.
'failing' indicates an anagram.
'to protect' means one lot of letters goes inside another.
'primarily' suggests taking the first letters.
The initial letter of 'canines' is 'c'.
'on'+'dentist'='ondentist'
'ondentist' is an anagram of 'disontent'.
'disontent' going around 'c' is 'DISCONTENT'.
(Other definitions for discontent that I've seen before include "Unhappiness" , "blighted Gloucester's winter" , "Now is the winter of our - (Richard III)" , "Dissatisfaction" , "Lack of satisfaction" .)