Canine upset principal in Trinity (7)
I believe the answer is:
godhead
'trinity' is the definition.
(I've seen this before)
'canine upset principal' is the wordplay.
'canine' becomes 'dog' (dog is a kind of canine).
'upset' shows that the letters should be reversed in order.
'principal' becomes 'head' (synonyms).
'dog' written backwards gives 'god'.
'god'+'head'='GODHEAD'
'in' acts as a link.
(Other definitions for godhead that I've seen before include "deity" , "Had doge partaken or divine nature?" , "Creator" , "Trinity" .)