Do but disbelieve (5)
I believe the answer is:
doubt
'disbelieve' is the definition.
(doubting is a kind of disbelieving)
'do but' is the wordplay.
'do but' can be anagrammed to 'DOUBT'.
However, I'm unsure how the anagram is indicated.
Can you help me to learn more?
(Other definitions for doubt that I've seen before include "Uncertainty about the truth" , "Question the truth or accuracy of" , "Lack of conviction" , "Misgiving" , "Be uncertain about or consider unlikely" .)