Archaic lilac occasionally produces bush (6)
I believe the answer is:
Here is my best explanation:
'bush' is the definition.
'archaic lilac occasionally' is the subsidiary indication.
'occasionally' means one should take alternating letters.
The alternating letters of 'archaiclilac' are 'acacia'.
'produces' is the link.
(Other definitions for acacia that I've seen before include "ornamental tree", "Tree or shrub with yellow or white flowers", "source of gum arabic", "Wattle genus", "Thorny shrub with yellow or white-flowers", "flowering plant", "Shrub of warm climates", "(Sub)tropical shrub with yellow or white flowers - wattle", "Wattle or Robinia" and "Subtropical shrub or tree".)