Plant a group of US agents to protect California (6)
I believe the answer is:
acacia
'plant' is the definition.
The answer is an example.
'a group of us agents to protect california' is the wordplay.
'group of us agents' becomes 'CIA' (Central Intelligence Agency in the US).
'to protect' is an insertion indicator (inserted letters are held or protected).
'california' becomes 'CA' (abbreviation).
'a'+'cia'='acia'
'acia' enclosing 'ca' is 'ACACIA'.
(Other definitions for acacia that I've seen before include "Spiny shrub with clusters of yellow or white flowers" , "Plant like wattle" , "Tree or shrub with yellow or white flowers" , "Tropical tree, inc. mimosa, wattle" , "Flowering tree/shrub of Australia and Africa" .)