Talk about root of lilaceous plant (7)
I believe the answer is:
parsley
'plant' is the definition.
(parsley is a kind of plant)
'talk about root of lilaceous' is the wordplay.
'talk' becomes 'parley' (to parley is to talk with an enemy).
'about' means one lot of letters goes inside another.
'root of' says to take the final letters (in a down clue, the letter at the bottom).
The final letter of 'lilaceous' is 's'.
'parley' placed around 's' is 'PARSLEY'.
(Other definitions for parsley that I've seen before include "Aromatic herb with flat or crinkly leaves" , "Herb (flat-leafed or curly)" , "Herbal garnish" , "Herb for garnishing" , "Cow -, fool's - and stone - are all plants" .)