Booked to be taciturn (8)
I believe the answer is:
reserved
This clue is a double definition.
'booked' is the first definition.
(similar in meaning)
'taciturn' is the second definition.
(thesaurus)
'to be' is the link.
(Other definitions for reserved that I've seen before include "without emotion" , "Held back, set aside" , "Shy - set aside" , "Retiring" , "Spoken for" .)