Clue: Questionable reading (hasty perhaps), an oath, not dialectal (9)
Answer: CURSENARY
Explanation: Definition: Questionable reading (hasty perhaps)
CURSE (oath) + NARY (not, in dialect) – cursenary (also seen as cursorary) is a questionable word in the original of *Henry V*, possibly intended to be "cursitory"="cursory"="hasty"