New expedition from east of London to secure Kentish garden centre? (7)
I believe the answer is:
nursery
'garden centre?' is the definition.
(I've seen this before)
I don't understand the rest of the clue.
(Other definitions for nursery that I've seen before include "Place where young plants are grown" , "Garden centre" , "An end of some tests" , "cannon" , "Where trees are tended" .)