Wise, quiet, like Gray’s forefathers. going by the books (7)
I believe the answer is:
prudent
'wise' is the definition.
(associated in meaning)
'quiet like gray's forefathers going by the books' is the wordplay.
'quiet like gray's forefathers' becomes 'prude' (I can't justify this - if you can you should believe this answer much more).
'going by' means one lot of letters go next to another.
'the books' becomes 'nt' (abbreviation for New Testament).
'prude'+'nt'='PRUDENT'
Can you help me to learn more?
(Other definitions for prudent that I've seen before include "Careful, circumspect" , "Careful and sensible" , "Acting with care for the future" , "Judicious" , "Thinking of the future" .)