Plant beginning to grow above ancient bar (9)
I believe the answer is:
goldenrod
'plant' is the definition.
(goldenrod is a kind of plant)
'beginning to grow above ancient bar' is the wordplay.
'beginning to' says to take the initial letters.
'above' says to put letters next to each other (in a down clue, some letters go above others).
'ancient' becomes 'olden' (as in the olden days).
'bar' becomes 'rod' (both can mean a pole or stick).
The initial letter of 'grow' is 'g'.
'g'+'olden'+'rod'='GOLDENROD'
(Other definitions for goldenrod that I've seen before include "Yellow-headed garden plant" , "Plant with tall spikes of yellow flowers" , "Yellow-flowered plant" , "Plant with spikes of tiny golden flowers" , "Yellow-spiked garden flower" .)