The phrase "a slightly dull" is correct and usable in written English.
It can be used to describe something that is somewhat uninteresting or lacking in excitement.
Example: "The presentation was a slightly dull affair, lacking the energy we had hoped for."
Alternatives: "somewhat boring" or "a bit tedious."
In the film, Biko talks like a slightly dull vicar from Suffolk.
Marginal observable difference was found in product colour for the various treatment combinations, with a slightly dull appearance, viz less white and less yellow compared with the control.
Hyundai Veloster hyundai-car.co.uk, from £18,000 The Veloster is a slightly dull drive but scores points for its looks.
Maybe a slightly dull future, but as your parents will tell you, there are worse fates than being bored occasionally.
What I lacked was something in between, a simply designed sharpener I could keep handy to bring a slightly dull knife back to life.
But there's a slightly dull anonymity to Krewella's vocals — there's no way to tell if the Yousaf sisters are great, or even interesting singers, because of all the effects.
And, in the grassland, is the least minor moth, a scarce, slightly dull brown, day-flying species.
Mr Kaine, a competent but slightly dull fellow with big eyebrows, even made Mr Obama's shortlist.
Mr Kerry was a reasonable but slightly dull fellow who owed much of his support from young voters to their contempt for Mr Bush.
Companion digital shows with a well-mannered, slightly dull air of decorous efficiency and cheerful modernity that encapsulates what you may regard as an ethos at large.
One could argue that Whitlock is the -Murray of the gymnastics world; he prefers a quiet, even slightly dull life compared with the celebrity career his good friend Louis has carved out.
