It grows on both sides of an upright (5)
I believe the answer is:
onion
'upright' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
'it grows on both sides of an' is the wordplay.
'it grows' becomes 'on' (I can't justify this - if you can you should believe this answer much more).
'both sides of' indicates putting letters inside (one word stretches round both sides of another).
'an' becomes 'i' (Roman numeral for one).
'on'+'on'='onon'
'onon' enclosing 'i' is 'ONION'.
Can you help me to learn more?
(Other definitions for onion that I've seen before include "Tear-inducing edible bulb" , "Strong tasting vegetable" , "Bulbous vegetable" , "Alliaceous plant" , "reason for tears" .)