Big plant needs large space outside (3)
I believe the answer is:
elm
'big plant' is the definition.
'elm' can be an answer for 'plant' (I have seen 'Perennial plant' mean 'elm' so perhaps 'plant' could also mean 'elm'). I'm unsure of the 'big' bit.
'large space outside' is the wordplay.
'large' becomes 'L' (eg in clothes sizes).
'space' becomes 'em' (in typography, a space as wide as a letter 'm').
'outside' is an insertion indicator (some letters go outside others).
'l' put inside 'em' is 'ELM'.
'needs' is the link.
Can you help me to learn more?
(Other definitions for elm that I've seen before include "I branch out" , "Wood in pelmet" , "Mel's tree" , "Timber tree" , "Tree of the genus Ulmus" .)