An undergarment one's upset about, showing wear (8)
I believe the answer is:
abrasion
'showing wear' is the definition.
(I've seen this before)
'an undergarment one's upset about' is the wordplay.
'an' becomes 'a'.
'undergarment' becomes 'bra' (I've seen this before).
'one's upset' becomes 'si' (I am not sure about this - if you are sure you should believe this answer much more).
'about' becomes 'on'.
'a'+'bra'+'si'+'on'='ABRASION'
Can you help me to learn more?
(Other definitions for abrasion that I've seen before include "Grazing" , "showing wear" , "Cut or scratch" , "Skin wound" , "scraping away" .)