Clearing large borders, exposed umbelliferous plant (5)
I believe the answer is:
sedum
'plant' is the definition.
(sedum is a kind of plant)
'clearing large borders exposed umbelliferous' is the wordplay.
'clearing large borders' says the answer is hidden in the clue (letters bordering the answer are cleared).
'SEDUM' is hidden within 'exposed umbelliferous'.
(Other definitions for sedum that I've seen before include "Fleshy-leaved plant related to stonecrop" , "Mused about the rock plant" .)