It grows up, not out on the left hand side (5)
I believe the answer is:
lupin
'left hand side' is the definition.
Although both the answer and definition are singular nouns, I can't understand how one could define the other.
'it grows up not out on' is the wordplay.
'it grows' becomes 'l' (I am not sure about this - if you are sure you should believe this answer much more).
'not out' becomes 'in' ('in' is the opposite to 'out').
'on' says to put letters next to each other.
'l'+'up'+'in'='LUPIN'
'the' acts as a link.
Can you help me to learn more?
(Other definitions for lupin that I've seen before include "Pea relative with tall spikes of flowers" , "Spiky flower" , "Fodder plant" , "Flower of pea family" , "Flowering plant" .)