Naughty baron with married friend breaking conventions? (10)
I believe the answer is:
abnormally
'breaking conventions?' is the definition.
(defying conventions or norms)
'naughty baron with married friend' is the wordplay.
'naughty' indicates anagramming the letters.
'with' says to put letters next to each other.
'married' becomes 'm' (abbreviation, e.g. in genealogy).
'friend' becomes 'ally' (an ally is a friend or supporter).
'baron' anagrammed gives 'abnor'.
'abnor'+'m'+'ally'='ABNORMALLY'
(Other definitions for abnormally that I've seen before include "unexpectedly" , "oddly" .)