The smelly pants? (9)
I believe the answer is:
halitosis
'the smelly pants?' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
This is all the clue.
Can you help me to learn more?
(Other definitions for halitosis that I've seen before include "repulsive pants?" , "offensive condition" , "Bad breath, smells unpleasant" , "Foul breath problem" , "Smelly breath" .)