Willing girl's modelled (8)
I believe the answer is:
disposed
'willing' is the definition.
('disposed' can be similar in meaning to 'willing')
'girl's modelled' is the wordplay.
'girl's' becomes 'dis'.
'modelled' becomes 'posed' (model and pose are synonyms).
'dis'+'posed'='DISPOSED'
(Other definitions for disposed that I've seen before include "of a mind" , "Got rid of" , "Placed suitably or in order" , "Ready or inclined to do something" , "Inclined, willing" .)