Scratchy things as with undergarments I've taken in (9)
I believe the answer is:
abrasives
'scratchy things' is the definition.
Although both the answer and definition are plural nouns, I don't see how one could define the other.
'as with undergarments i've taken in' is the wordplay.
'with' indicates putting letters inside.
'undergarments' becomes 'bras' (bra is a kind of undergarment).
'taken in' means one lot of letters goes inside another.
'as' going around 'bras' is 'abrass'.
'abrass' enclosing 'ive' is 'ABRASIVES'.
Can you help me to learn more?
(Other definitions for abrasives that I've seen before include "Scratchy materials" , "Substances used for grinding or polishing" .)