I f you use 'a' it is a headword in Chambers, if you use 'u', it isn't, even though it is a very common derivative of a headword. Anyone know what Doc means by 'inflected' in this context?
I have interpreted it as meaning either the plural of a noun or the third person singular of a verb. If that is right the U version is correct while the A version is not as that is given as a straightforward abbreviation.