Slender volume bound in steel, unexpectedly (6)
I believe the answer is:
svelte
'slender' is the definition.
(synonyms)
'volume bound in steel unexpectedly' is the wordplay.
'volume' becomes 'v' (abbreviation for volume in book titles).
'bound in' indicates putting letters inside.
'unexpectedly' is an anagram indicator.
'steel' with letters rearranged gives 'selte'.
'v' put into 'selte' is 'SVELTE'.
(Other definitions for svelte that I've seen before include "Of delicate or slender build" , "Sylphlike" , "Gracefully slim" , "Slim and attractive" , "Slender - refined" .)