Quell doubt possibly, by voicing this incantation (6)
I believe the answer is:
dispel
'quell' is the definition.
Although both the answer and definition are verbs in their base form, I cannot understand how they can define each other.
'doubt possibly by voicing this incantation' is the wordplay.
'doubt possibly' becomes 'di' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'by' says to put letters next to each other.
'voicing' indicates a 'sounds like' (homophone) clue (I've seen 'voiced' mean this).
'this incantation' becomes 'spell' (I've seen this before).
'spell' is a homophone of 'spel'.
'di'+'spel'='DISPEL'
Can you help me to learn more?
(Other definitions for dispel that I've seen before include "Lisped (anag)" , "'Disperse, clear away (6)'" , "Banish, drive away" , "Make (something) disappear" , "Get rid of" .)