Important: arm this police unit (7,6)
I believe the answer is:
special branch
'this police unit' is the definition.
(police department)
'important arm' is the wordplay.
'important' becomes 'special' (synonyms).
'arm' becomes 'branch' (synonyms).
'special'+'branch'='SPECIAL BRANCH'
(Other definitions for special branch that I've seen before include "Division of police dealing with political security" , "Police responsible for political security" , "Police department dealing with political security" , "officers" , "counter-terrorist unit?" .)