Period of time in which to embed a plant (8)
I believe the answer is:
centaury
'plant' is the definition.
(centaury is a kind of plant)
'period of time in which to embed a' is the wordplay.
'period of time' becomes 'century' (century is a kind of period of time).
'in which to embed' indicates putting letters inside.
'century' placed around 'a' is 'CENTAURY'.
(Other definitions for centaury that I've seen before include "flowering plant" , "Pink-flowered plant of gentian family" .)