Burning painting outside lair (6)
I believe the answer is:
ardent
'burning' is the definition.
(both can mean passionate)
'painting outside lair' is the wordplay.
'painting' becomes 'art' (painting is an art).
'outside' indicates putting letters inside (some letters go outside others).
'lair' becomes 'den' (synonyms).
'art' placed around 'den' is 'ARDENT'.
(Other definitions for ardent that I've seen before include "Passionate, fervent" , "Burning, passionate" , "passion" , "Enthusiastic or passionate" , "Burning with emotion" .)