Heartless book by the ultra-fashionable (6)
I believe the answer is:
brutal
'heartless' is the definition.
(thesaurus)
'book by the ultra-fashionable' is the wordplay.
'book' becomes 'b' (abbreviation for book).
'by' is a charade indicator (letters next to each other).
'fashionable' indicates anagramming the letters (letters can be fashioned into something else).
'ultra' is an anagram of 'rutal'.
'b'+'rutal'='BRUTAL'
(Other definitions for brutal that I've seen before include "cruelly savage" , "Cruel, heartless" , "Very cruel, merciless" , "Harsh, merciless" , "Rough and cruel" .)