From The Enigma
NPL Magazine - June, 2012
SCARAB

[The two answers to this flat differ by a single letter.]

I can't use bleach to clean my tub,
It makes my bronchi burn,
But I'm afraid that something _ _ _ _ _ _
Will let the _ _ _ _ _ _ return.

HINT:
The last letters of each word are the differing letters.

Answers:

MILDER
MILDEW