Put one's foot down on healthy Veronica plant (9)
I believe the answer is:
speedwell
'veronica plant' is the definition.
(I know that veronica plant can be written as speedwell)
'put one's foot down on healthy' is the wordplay.
'put one's foot down' becomes 'speed' (I can't justify this - if you can you should give a lot more credence to this answer).
'on' means one lot of letters go next to another (in a down clue, letters appear on others).
'healthy' becomes 'well' (similar in meaning).
'speed'+'well'='SPEEDWELL'
Can you help me to learn more?
(Other definitions for speedwell that I've seen before include "Flowering plant" , "Small blue flower of the genus veronica" , "bloomer?" , "Small creeping herbaceous plant with blue or pink flowers" , "Small plant, veronica" .)