A cut from knife that's sharp (6)
I believe the answer is:
clever
'sharp' is the definition.
(I've seen this before)
'a cut from knife' is the wordplay.
'cut from' indicates named letters should be taken away.
'knife' becomes 'cleaver' (cleaver is a kind of knife).
'cleaver' with 'a' taken away is 'CLEVER'.
'that's' is the link.
(Other definitions for clever that I've seen before include "Smart or quick at learning" , "Smar and canny" , "Bright - ingenious" , "Quick-witted, smart" , "Skilful, adroit" .)