Plant one on both sides (5)
I believe the answer is:
onion
'plant one' is the definition.
The definition and answer can be both plants as well as being singular nouns.
Maybe they are linked in a way I don't understand?
'on both sides' is the wordplay.
'both sides' becomes 'ion' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'on'+'ion'='ONION'
Can you help me to learn more?
(Other definitions for onion that I've seen before include "Eye-watering vegetable" , "Tear-jerking bulb" , "Tear-inducing edible bulb" , "Hot dog topping" , "Plant with edible bulb" .)