Knockout stuff - cool, extremely raffish tailored fashion (10)
I believe the answer is:
chloroform
'knockout stuff' is the definition.
'cool extremely raffish tailored fashion' is the wordplay.
'extremely' suggests removing the centre (only the extremes of the word are used).
'tailored' indicates an anagram (the letters are customised ).
'fashion' becomes 'form' (both can mean to shape something for a purpose).
'raffish' with its middle taken out is 'rh'.
'cool'+'rh'='coolrh'
'coolrh' with letters rearranged gives 'chloro'.
'chloro'+'form'='CHLOROFORM'
(Other definitions for chloroform that I've seen before include "Old anaesthetic" , "Sweet-smelling anaesthetic" , "Former anaesthetic" , "Solvent" .)