Inaugurate plant that's key to discovering riches (4,6)
I believe the answer is:
open sesame
'key to discovering riches' is the definition.
(phrase from 'Ali Baba and the Forty Thieves')
'inaugurate plant' is the wordplay.
'inaugurate' becomes 'open'.
'plant' becomes 'sesame' (sesame is a kind of plant**).
'open'+'sesame'='OPEN SESAME'
'that's' acts as a link.
(Other definitions for open sesame that I've seen before include "Magic password" , "free entry" , "Magical words" , "a very successful means of achieving a result" , "Words of Ali Baba" .)