Republican invested in key American plant (8)
I believe the answer is:
marjoram
'plant' is the definition.
'republican invested in key american' is the wordplay.
'republican' becomes 'r' (abbreviation).
'invested in' means one lot of letters goes inside another.
'key' becomes 'major'.
'american' becomes 'am' (abbreviation in some contexts).
'r' placed into 'major' is 'marjor'.
'marjor'+'am'='MARJORAM'
(Other definitions for marjoram that I've seen before include "Aromatic herb" , "Herb used in soups, stews etc" , "Kind of herb" , "Herb, maybe oregano" , "Arm major (anag)" .)