Fools lower energy a bit in oppressive heat (9)

I haven't cracked this one yet — but the brilliant team at Times for the Times have:

Another one which tooke me too long; I wanted the answer to be MUGGINESS for oppressive heat, very topical, but the crossing BASIS had to be right; then I saw you could move the E down one space to get MUGGINSES. Not a word I’ve ever used but I knew MUGGINS which is a singular fool so the plural could be MUGGINSES.