Sincere about source of contentment being peace (5)
I believe the answer is:
truce
'peace' is the definition.
(truce is a kind of peace)
'sincere about source of contentment' is the wordplay.
'sincere' becomes 'true' ('true' can be a synonym of 'sincere').
'about' indicates putting letters inside.
'source of' suggests taking the first letters.
The initial letter of 'contentment' is 'c'.
'true' placed around 'c' is 'TRUCE'.
'being' is the link.
(Other definitions for truce that I've seen before include "Agreement to stop battle" , "Agreed ceasefire" , "Agreement to stop fighting temporarily" , "Cruet (anag.)" , "bit of peace" .)