Knife fight about cash register that's short (8)
I believe the answer is:
stiletto
'knife' is the definition.
(I know that stiletto is a type of dagger)
'fight about cash register that's short' is the wordplay.
'fight' becomes 'set-to' (set-to can mean a fight or argument).
'about' is an insertion indicator.
'cash register' becomes 'till' (I've seen this before).
'that's short' means to remove the last letter.
'till' with its final letter taken off is 'til'.
'setto' placed around 'til' is 'STILETTO'.
(Other definitions for stiletto that I've seen before include "Stabbing dagger or high heel" , "Bedspread" , "part of boot?" , "Small dagger with a tapered blade" , "Possible cause of stabbing pains" .)