Initially heavy duty keeps one sharp (5)
I believe the answer is:
honed
'sharp' is the definition.
'initially heavy duty keeps one' is the wordplay.
'initially' says to take the initial letters.
'keeps' is an insertion indicator.
The first letters of 'heavy duty' is 'hd'.
'hd' enclosing 'one' is 'HONED'.
(Other definitions for honed that I've seen before include "Refined" , "Sharpened with a stone" , "Perfected" .)