Plant officer eats right before a mass (8)
I believe the answer is:
marjoram
'plant' is the definition.
'officer eats right before a mass' is the wordplay.
'officer' becomes 'major' (major is a kind of officer).
'eats' indicates putting letters inside.
'right' becomes 'r' (common abbreviation).
'before' says to put letters next to each other.
'mass' becomes 'm' (symbol used in Physics e.g. f=ma).
'major' placed around 'r' is 'marjor'.
'marjor'+'a'+'m'='MARJORAM'
(Other definitions for marjoram that I've seen before include "Aromatic plant used for seasoning food" , "Aromatic herb" , "Food seasoning also called oregano" , "Herb, maybe oregano" , "Arm major (anag)" .)