Certainly, carrying knife one goes in for sharpness (6)
I believe the answer is:
acuity
'sharpness' is the definition.
(synonyms)
'certainly carrying knife one goes in' is the wordplay.
'certainly' becomes 'ay' (variant spelling of 'aye').
'carrying knife' becomes 'cuit' (I can't justify this - if you can you should give a lot more credence to this answer).
'one goes in' means one lot of letters goes inside another.
'ay' going around 'cuit' is 'ACUITY'.
'for' is the link.
Can you help me to learn more?
(Other definitions for acuity that I've seen before include "Sharp-sightedness, keenness" , "Keenness of perception" , "Sharpness (e.g. of mind)" , "Sharpness of vision or a quick intelligence" , "Keenness of vision" .)