Suspect (5)
'suspect' is the definition.
(I know that doubt is a more specific form of the action suspect)
This is the entire clue.
(Other definitions for doubt that I've seen before include "Be uncertain about or consider unlikely" , "Have reservations or be uncertain about" , "Feel uncertain about" , "Uncertainty, unsureness" , "Disbelieve (someone)" .)
'suspect' is the definition.
(I know that suspect can be written as mistrust)
This is all the clue.
(Other definitions for mistrust that I've seen before include "Lack of belief (in someone)" , "Regard with suspicion" , "Suspect (someone)" , "Have no confidence in (someone)" , "reservations" .)