Attend dressed in style? That's not unknown (6)
I believe the answer is:
listen
'attend' is the definition.
(thesaurus)
'dressed in style? that's not unknown' is the wordplay.
'dressed' indicates an anagram ('dress' can mean 'prepare').
'that's not' is a deletion indicator.
'unknown' becomes 'y' (algebra).
'in'+'style'='instyle'
'instyle' with 'y' taken out is 'instle'.
'instle' is an anagram of 'LISTEN'.
(Other definitions for listen that I've seen before include "16 (anag.)" , "Tinsel (anag.)" , "Try to hear (something)" , "Pay heed (to advice)" , "Be silent in order to hear" .)