Turned up, primarily dropping even knickers (au naturel) (5)
I believe the answer is:
naked
'au naturel' is the definition.
('naked' can be a synonym of 'au naturel')
'turned up primarily dropping even knickers' is the wordplay.
'turned' becomes 'na' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'primarily' indicates taking the first letters.
The initial letters of 'dropping even knickers' is 'dek'.
'dek' reversed gives 'ked'.
'na'+'ked'='NAKED'
(Other definitions for naked that I've seen before include "Without clothes" , "In one's birthday suit" , "Nude" , "Without protection" , "being exposed" .)