Like some knickers, perhaps -- though clearly not new! (5)
I believe the answer is:
satin
'like some knickers perhaps though clearly' is the definition.
I can't judge whether this defines the answer.
'not new' is the wordplay.
'not' becomes 'sati' (I can't explain this - if you can you should believe this answer much more).
'new' becomes 'n'.
'sati'+'n'='SATIN'
Can you help me to learn more?
(Other definitions for satin that I've seen before include "Smooth glossy fabric" , "Smooth and silky" , "Stain on silky cloth" , "Lustrous fabric" , "Stain found in cloth" .)