Knife fight about it upset student (8)
I believe the answer is:
stiletto
'knife' is the definition.
(stiletto is a kind of knife)
'fight about it upset student' is the wordplay.
'fight' becomes 'setto' (set-to is a kind of fight**).
'about' indicates putting letters inside.
'upset' is a reversal indicator.
'student' becomes 'l'.
'it' back-to-front is 'ti'.
'ti'+'l'='til'
'setto' enclosing 'til' is 'STILETTO'.
(Other definitions for stiletto that I've seen before include "One may have a stab" , "Type of heel" , "Small dagger and high narrow heel" , "High-heeled shoe" , "Short dagger; shoe heel" .)