Lustrous, quiet, fashionable and unknown (5)
I believe the answer is:
shiny
'lustrous' is the definition.
(thesaurus)
'quiet fashionable and unknown' is the wordplay.
'quiet' becomes 'sh'.
'fashionable' becomes 'in' ('in' can be similar in meaning to 'fashionable').
'and' says to put letters next to each other.
'unknown' becomes 'y' (algebra).
'sh'+'in'+'y'='SHINY'
(Other definitions for shiny that I've seen before include "Gleaming" , "Bright" , "Smooth and polished" , "Lustrous, glossy" , "Smooth and reflective" .)