The phrase "supposed expert" is correct and can be used in written English.
It is often used to refer to someone who is believed to be an expert, but their qualifications or expertise may be questionable. Example: The supposed expert on climate change turned out to have no scientific background and was later exposed as a fraud.
He was teaching me things I didn't know, even though I was the supposed expert".
Mat didn't understand why policymakers were failing to act on this supposed expert consensus and began to research the subject for himself.
Immigration officials supported asylum decisions with the testimony of a supposed expert from the Swedish firm Sprakab, who has been accused of not holding the qualifications he claimed to, and who has a conviction for drug smuggling.
His life turned around when he (somewhat incongruously) began writing best-selling romance novels that have made him a minor literary star and a supposed expert on love, about which he actually knows next to nothing.
The supposed expert on Islam was talking about freedom of speech in the wake of the Charlie Hebdo shootings and referred to Mehdi Hasan, the political director at HuffingtonPost UK, as "Ahmed".
The funniest, and still most topical, scene shows the restored Jack being investigated for lingering signs of lunacy by a supposed expert: Jack's attacks on immigrant barbarians are seen as an instant mark of his sanity, but the clincher comes when he and his examiner join together in an old-boy rendering of the Eton Boating Song.
But these supposed experts are deluding themselves if they think the question has been resolved.
Even more disturbing, specialist prosecutors – supposed experts in the field – were used on the case.
Wine tasting was previously mentioned, and how studies show the supposed experts are actually very inconsistent.
Clearly, the supposed experts who predicted the imminent disintegration of the eurozone have been proved wrong.
These parents believed the authority figures: the police, prosecutors, supposed experts.
