Bush can stop former head of state (8)
I believe the answer is:
lavender
'bush' is the definition.
(lavender is a kind of bush)
'can stop former head of state' is the wordplay.
'can' becomes 'lav' (synonyms).
'stop' becomes 'end' (synonyms).
'former head of state' becomes 'ER' (Elizabeth Regina, ie Queen Elizabeth).
'lav'+'end'+'er'='LAVENDER'
(Other definitions for lavender that I've seen before include "Light bluish-purple" , "Plant used in perfumery" , "Aromatic shrub related to mint" , "Plant with aromatic purple flowers" , "Shrub with fragrant purple flowers, used to scent linen" .)