Injuring knee lower, dagger's first to go in — is that sharp practice? (10)
I believe the answer is:
needlework
'that sharp practice?' is the definition.
(practice involving sharp objects)
'injuring knee lower dagger's first to go in' is the wordplay.
'injuring' indicates anagramming the letters (I've seen 'injured' mean this).
'dagger's first' becomes 'd' (1st letter of 'dagger').
'to go in' means one lot of letters goes inside another.
'knee'+'lower'='kneelower'
'kneelower' is an anagram of 'neelework'.
'neelework' going around 'd' is 'NEEDLEWORK'.
'is' acts as a link.
(Other definitions for needlework that I've seen before include "putting in stitches" , "Sewing - embroidery" , "craft" , "sewers do that" , "This could repair it" .)