Integrity of perfect pen (7)
I believe the answer is:
honesty
'integrity' is the definition.
(I've seen this before)
'perfect pen' is the wordplay.
'perfect' becomes 'hone' (synonyms).
'pen' becomes 'sty' (sty is a kind of pen).
'hone'+'sty'='HONESTY'
'of' acts as a link.
(Other definitions for honesty that I've seen before include "Uprightness" , "Integrity" , "Plant; sincerity" , "Plant; truthfulness" , "Probity; plant" .)