Crossword Genius

On the allotment, one on both sides? (5)

Ross

I believe the answer is:

onion

I'm a little stuck... Click here to teach me more about this clue!

'one on both sides?' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how they can define each other.

'on the allotment' is the wordplay.
'the allotment' becomes 'ion' (I can't explain this - if you can 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 "Type of allium" , "Plant with edible bulb" , "veggie" , "Bulb used as a vegetable" , "Pungent vegetable" .)

I've seen this clue in the Evening Standard.
Want a hint initially instead of a full solution? Install my app