Archaic lilac occasionally produces bush (6)
I believe the answer is:
acacia
'bush' is the definition.
'archaic lilac occasionally' is the wordplay.
'occasionally' means one should take alternating letters.
'archaic'+'lilac'='archaiclilac'
The alternating letters of 'archaiclilac' are 'ACACIA'.
'produces' acts as a link.
(Other definitions for acacia that I've seen before include "Spiny tree or shrub" , "(Sub)tropical shrub with yellow or white flowers - wattle" , "Tree (wattle or robinia)" , "Wattle that grows" , "Tropical shrub" .)