Disprove source of false doctrine (6)
I believe the answer is:
belief
'doctrine' is the definition.
(belief is a kind of doctrine)
'disprove source of false' is the wordplay.
'disprove' becomes 'belie' (I've seen this in another clue).
'source of' indicates taking the first letters (the start - or source - of the word).
The first letter of 'false' is 'f'.
'belie'+'f'='BELIEF'
(Other definitions for belief that I've seen before include "Creed or opinion" , "Firm conviction" , "Faith; confidence" , "Tenet, creed" , "Faith, credence" .)