Knife fight involving cash register that's short (8)
I believe the answer is:
stiletto
'knife' is the definition.
(stiletto is a kind of knife)
'fight involving cash register that's short' is the wordplay.
'fight' becomes 'setto' (set-to can mean a fight or argument).
'involving' indicates putting letters inside.
'cash register' becomes 'till' (I've seen this in another clue).
'that's short' means to remove the last letter.
'till' with its final letter removed is 'til'.
'setto' placed around 'til' is 'STILETTO'.
(Other definitions for stiletto that I've seen before include "Stabbing dagger or high heel" , "Long dagger" , "Dagger with slender blade" , "A short knife, dagger or heel" , "Dagger; tapered heel" .)