Cut letter sharp (6)
I believe the answer is:
severe
'sharp' is the definition.
(I've seen this before)
'cut letter' is the wordplay.
'cut' becomes 'sever' (to sever is to cut or split).
'letter' becomes 'e' ().
'sever'+'e'='SEVERE'
(Other definitions for severe that I've seen before include "Plain, hard" , "Harsh - extreme" , "Stern, austere" , "Harsh; very plain" , "Very plain in style or appearance" .)