According to the book that's reserved (6)
I believe the answer is:
formal
'reserved' is the definition.
(similar in meaning)
'according to the book' is the wordplay.
I cannot quite understand how this works, but
'to' could be 'for' and 'for' is found in the answer.
The remaining letters 'mal' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
'that's' is the link.
Can you help me to learn more?
(Other definitions for formal that I've seen before include "It's required by rule" , "Conforming to accepted rules or customs" , "Befitting a person in authority" , "Conforming to accepted customs, say dress" , "Ceremonious or official" .)