Believes book about fungal growth (7)
I believe the answer is:
accepts
'believes' is the definition.
(I've seen this before)
'book about fungal growth' is the wordplay.
'book' becomes 'acts' (book of the Bible).
'about' is an insertion indicator.
'fungal growth' becomes 'cep'.
'acts' enclosing 'cep' is 'ACCEPTS'.
(Other definitions for accepts that I've seen before include "Receives" , "Says yes" , "Believes" , "Takes on board" , "Takes (an offer)" .)