Gets book about mushroom (7)
I believe the answer is:
accepts
'gets' is the definition.
(accepting is a kind of getting)
'book about mushroom' is the wordplay.
'book' becomes 'acts' (I've seen this before).
'about' indicates putting letters inside.
'mushroom' becomes 'cep' (Boletus edulis - an edible mushroom).
'acts' placed around 'cep' is 'ACCEPTS'.
(Other definitions for accepts that I've seen before include "Receives" , "Takes (an offer)" , "Consents to receive" , "Admits" , "Believes" .)