Plant book Maria's edited covers osmosis at the beginning (8)
I believe the answer is:
ambrosia
'plant' is the definition.
(ambrosia is a kind of plant)
'book maria's edited covers osmosis at the beginning' is the wordplay.
'book' becomes 'b'.
'edited' indicates an anagram.
'covers' is an insertion indicator.
'at the beginning' says to take the initial letters.
The first letter of 'osmosis' is 'o'.
'b'+'marias'='bmarias'
'bmarias' with letters rearranged gives 'ambrsia'.
'ambrsia' going around 'o' is 'AMBROSIA'.
(Other definitions for ambrosia that I've seen before include "Bee bread" , "Gods' dinner?" , "fine fuel" , "food from the deli?" , "Nectar of the gods" .)