Presumed half-supine model would be given first dress (8)
I believe the answer is:
supposed
'presumed' is the definition.
(thesaurus)
'model would be given first dress' is the wordplay.
'model' becomes 'pose' (model can mean to pose).
'would' becomes 'sup' (I can't justify this - if you can you should give a lot more credence to this answer).
'be given' is a charade indicator (letters next to each other).
'first dress' becomes 'd' (sst letter of 'dress').
'pose' put after 'sup' is 'suppose'.
'suppose'+'d'='SUPPOSED'
'half-supine' acts as a link.
This may not be right. It may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for supposed that I've seen before include "Assumed to be true or thought probable" , "Reckoned, took to be the case" , "Imagined, presumed" , "As expected" , "Imagined; assumed" .)