Still endlessly upset when cut (4)
I believe the answer is:
slit
'cut' is the definition.
(I know that slit is a type of cut)
'still endlessly upset' is the wordplay.
'endlessly' means to remove the last letter.
'upset' indicates an anagram.
'still' with its final letter taken away is 'stil'.
'stil' with letters rearranged gives 'SLIT'.
'when' acts as a link.
(Other definitions for slit that I've seen before include "Long narrow opening" , "Long narrow cut" , "Long straight cut" , "Narrow straight cut or opening" , "Narrow aperture" .)