Plant that's last in herbaceous border (5)
I believe the answer is:
sedge
'plant' is the definition.
(sedge is a plant)
'last in herbaceous border' is the wordplay.
'last in' says to take the final letters.
'border' becomes 'edge' (synonyms).
The last letter of 'herbaceous' is 's'.
's'+'edge'='SEDGE'
'that's' acts as a link.
(Other definitions for sedge that I've seen before include "Waterside plant" , "Grass-like plant growing in marshes" , "Wetland grass-like plant" , "plant with triangular stems" , "Herons" .)