Sharpens head of tool being blunt (6)
I believe the answer is:
honest
'being blunt' is the definition.
(blunt can mean honest or frank)
'sharpens head of tool' is the wordplay.
'sharpens' becomes 'hones' (honing is a kind of sharpening).
'head of' suggests taking the first letters.
The initial letter of 'tool' is 't'.
'hones'+'t'='HONEST'
(Other definitions for honest that I've seen before include "Without dissimulation or guile" , "Upright and straightforward" , "Honorable and sincere" , "Genuine and trustworthy" , "Genuine; fair" .)