It's hard to cut a key (6)
I believe the answer is:
severe
'it's hard to' is the definition.
(I've seen this before)
'cut a key' is the wordplay.
'cut' becomes 'sever' (to sever is to cut or split).
'a key' becomes 'E' (musical key).
'sever'+'e'='SEVERE'
(Other definitions for severe that I've seen before include "Arduous - dangerous" , "Austere, harsh (6)" , "Strict, stark" , "Drastic" , "Serious or strict" .)