Perhaps painting outside of study with heartless lady fervently (8)
I believe the answer is:
ardently
'fervently' is the definition.
(thesaurus)
'perhaps painting outside of study with heartless lady' is the wordplay.
'perhaps painting' becomes 'art' (painting is a kind of art).
'outside' means one lot of letters goes inside another.
'of study' becomes 'den' (I can't justify this - if you can you should believe this answer much more).
'with' says to put letters next to each other.
'heartless' means to remove the middle letters.
'lady' with its middle taken out is 'ly'.
'art' going around 'den' is 'ardent'.
'ardent'+'ly'='ARDENTLY'
(Other definitions for ardently that I've seen before include "with zeal" , "full of enthusiasm?" , "In a very passionate way" , "Passionately" , "With passion" .)