In canvas, Constable ultimately finds belief (5)
I believe the answer is:
tenet
'belief' is the definition.
('tenet' can be a synonym of 'belief')
'in canvas constable ultimately' is the wordplay.
'in' is an insertion indicator.
'canvas' becomes 'tent' (I've seen this in other clues).
'ultimately' suggests the final letters.
The last letter of 'constable' is 'e'.
'tent' placed around 'e' is 'TENET'.
'finds' acts as a link.
(Other definitions for tenet that I've seen before include "Teaching" , "Doctrine held to be true" , "That's the law" , "Doctrine; principle held" , "you may believe in it" .)