Long borders made primarily for a large plant (9)
I believe the answer is:
pimpernel
'plant' is the definition.
(pimpernel is a kind of plant)
'long borders made primarily for a large' is the wordplay.
'long' becomes 'pine' (to pine for someone is to long for them).
'borders' means one lot of letters goes inside another (inserted letters are bordered by others).
'primarily' indicates taking the first letters.
'for a' becomes 'per' ('per' can mean 'for each' person or 'for a' person).
'large' becomes 'L' (eg in clothes sizes).
The initial letter of 'made' is 'm'.
'm'+'per'='mper'
'pine' going around 'mper' is 'pimperne'.
'pimperne'+'l'='PIMPERNEL'
(Other definitions for pimpernel that I've seen before include "Once a much sought-after" , "The Scarlet ..., aka Sir Percy Blakeney" , "Small scarlet flower" , "of the English aristocracy once sought after in Paris?" , "Baroness Orczy's scarlet flower?" .)