Mournful sitter modelled (6)
I believe the answer is:
triste
'mournful' is the definition.
(I've seen this in another clue)
'sitter modelled' is the wordplay.
'modelled' indicates anagramming the letters (letters modelled or shaped).
'sitter' is an anagram of 'TRISTE'.
(Other definitions for triste that I've seen before include "Old word for sad" , "Valse -- (Sibelius)" , "Sad (arch)" , "Sad in France" .)