Spray shrub briefly with last of insecticide (7)
I believe the answer is:
syringe
'spray' is the definition.
(syringing is a kind of spraying)
'shrub briefly with last of insecticide' is the wordplay.
'shrub' becomes 'syringa' (type of shrub).
'briefly' means to remove the last letter.
'with' says to put letters next to each other.
'last of' says to take the final letters.
The final letter of 'insecticide' is 'e'.
'syringa' with its final letter removed is 'syring'.
'syring'+'e'='SYRINGE'
(Other definitions for syringe that I've seen before include "Means of injection" , "Injection device" , "Instrument for injecting fluids" , "What's used for injections" , "Clean out (ears)" .)