In fact, it's a nasty cut, friend (8)
I believe the answer is:
actually
'in fact' is the definition.
(synonyms)
'a nasty cut friend' is the wordplay.
'nasty' indicates anagramming the letters (a nasty or wrong spelling).
'friend' becomes 'ally' (an ally is a friend or supporter).
'cut' is an anagram of 'ctu'.
'a'+'ctu'+'ally'='ACTUALLY'
'it's' acts as a link.
(Other definitions for actually that I've seen before include "Really, in fact" , "As a matter of fact" , "Really existing" , "In reality" , "A lay cult may be existing in fact" .)