Greek character carrying book that's forbidden (4)
I believe the answer is:
tabu
'forbidden' is the definition.
('tabu' can be a synonym of 'forbidden')
'greek character carrying book' is the wordplay.
'greek character' becomes 'tau' (letter from the Greek alphabet).
'carrying' means one lot of letters goes inside another.
'book' becomes 'b'.
'tau' going around 'b' is 'TABU'.
'that's' is the link.
(Other definitions for tabu that I've seen before include "Social prohibition" , "delicate matter?" , "Ban resulting from social custom" , "Forbidden (variant spelling)" , "Forbidden (Alt sp.)" .)