Censor of old books turned on roughly (4)
I believe the answer is:
cato
'censor' is the definition.
(ancient Roman censor)
'old books turned on roughly' is the wordplay.
'old books' becomes 'ot' (Old Testament).
'turned' shows that the letters should be reversed in order.
'on' means one lot of letters go next to another.
'roughly' becomes 'ca' (abbreviation for circa).
'ot' backwards is 'to'.
'to' after 'ca' is 'CATO'.
'of' is the link.
(Other definitions for cato that I've seen before include "Ancient Roman censor" , "Senator" , "Marcus Porcius - , Roman politician, censor" , "Roman statesman" , "classical writer" .)