Arboriculturist may cherish this security service (7,6)
I believe the answer is:
special branch
'arboriculturist may cherish this security service' is the definition.
The definition suggests a singular noun which matches the answer.
This is the entire clue.
Can you help me to learn more?
(Other definitions for special branch that I've seen before include "counter-terrorist unit?" , "Police responsible for political security" , "Police department dealing with political security" , "Division of police dealing with political security" , "officers" .)