Ultimately that unwanted plant is a border flower (5)
I believe the answer is:
tweed
'a border flower' is the definition.
(I've seen this before)
'ultimately that unwanted plant' is the wordplay.
'ultimately' suggests the final letters.
'unwanted plant' becomes 'weed' (I've seen this in another clue).
The final letter of 'that' is 't'.
't'+'weed'='TWEED'
'is' acts as a link.
(Other definitions for tweed that I've seen before include "Woollen material" , "Woollen cloth - Scotland/England border river" , "Flow" , "Harris cloth" , "Scottish and English river" .)