Sharpen pen, being blunt perhaps (7)
I believe the answer is:
honesty
'being blunt perhaps' is the definition.
(to be blunt is to be candid or honest)
'sharpen pen' is the wordplay.
'sharpen' becomes 'hone' (honing is a kind of sharpening).
'pen' becomes 'sty' (sty is a kind of pen).
'hone'+'sty'='HONESTY'
(Other definitions for honesty that I've seen before include "Plant; truthfulness" , "that's easily cultivated" , "Straightforwardness" , "It's said to be the best policy" , "Fairness" .)