Unpretentiousness of style with pen (7)
I believe the answer is:
modesty
'unpretentiousness' is the definition.
(thesaurus)
'style with pen' is the wordplay.
'style' becomes 'mode' ('mode' can be a synonym of 'style'**).
'with' is a charade indicator (letters next to each other).
'pen' becomes 'sty' (sty is a kind of pen).
'mode'+'sty'='MODESTY'
'of' acts as a link.
(Other definitions for modesty that I've seen before include "Unassumingness" , "Quality of being unpretentious" , "lack of vanity" , "Diffidence" , "Unpretentious style" .)