There is a certain common word in English language (say XYZ) such that prefixing it with a letter A creates a new word (AXYZ) that is a plural of another valid English word.

Find it - preferably with pen, paper and your knowledge only.

There is a possibility for several answers...

(In reply to Funny... (spoilers) by Charlie)

I think the A must be taken literally in order to make this a D3 puzzle.  Otherwise, for instance, here are many letters that can be prefixed to ITS in order to make a plural.

Posted by Steve Herman on 2017-07-31 11:30:24