Fine lubricant for sword (4)
I believe the answer is:
foil
'sword' is the definition.
(foil is a kind of sword)
'fine lubricant' is the wordplay.
'fine' becomes 'f' (abbreviation in pencil classifications).
'lubricant' becomes 'oil' (I've seen this before).
'f'+'oil'='FOIL'
'for' acts as a link.
(Other definitions for foil that I've seen before include "Check" , "Prevent from succeeding" , "Repulse" , "shiny paper" , "Piece of thin and flexible sheet metal" .)