Sincerity as a matter of fact (5)
I believe the answer is:
truth
'fact' is the definition.
(I know that truth is a type of fact)
'sincerity as a matter' is the wordplay.
I cannot quite see how this works, but
'matter' could be 'th' (Th is an example) and 'th' is present in the answer.
This explanation may well be incorrect...
'of' acts as a link.
Can you help me to learn more?
(Other definitions for truth that I've seen before include "Candour" , "Beauty is (said Keats)" , "Honesty" , "Belief accepted as correct" , "Gospel" .)