Boss of many a plant (6)
I believe the answer is:
master
'boss' is the definition.
(I know that master is a type of ruler)
'many a plant' is the wordplay.
'many a' becomes 'm' (this might be a standard abbreviation I've not previously seen).
'plant' becomes 'aster' (aster is a kind of plant).
'm'+'aster'='MASTER'
'of' acts as a link.
Can you help me to learn more?
(Other definitions for master that I've seen before include "An old one is a great European painter" , "The boss" , "One has consummate skill" , "Teacher to gain control" , "Instructor" .)