Queen's plant (3)
I believe the answer is:
ers
'plant' is the definition.
Although both the answer and definition are nouns, they are not in the same form of the noun.
'queen's' is the wordplay.
I cannot really understand how this works, but
'queen's' could be 'rs' (abbreviation for regina, e.g. in ER ) and 'rs' is located in the answer.
A single letter 'e' remains which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Another definition for ers that I've seen is " Bitter vetch".)