Shapeless half-nude with blouse undone (8)
I believe the answer is:
nebulous
'shapeless' is the definition.
(thesaurus)
'half-nude with blouse undone' is the wordplay.
'half' indicates taking half.
'with' means one lot of letters go next to another.
'undone' indicates anagramming the letters.
'nude' with half the letters taken is 'nu'.
'nu' put after 'blouse' is 'blousenu'.
'blousenu' with letters rearranged gives 'NEBULOUS'.
(Other definitions for nebulous that I've seen before include "Hazy and cloudlike" , "Hazy, like a cloud" , "Vague and formless" , "Cloudlike; vague" , "'Cloudy, lacking definite form (8)'" .)