You do so as a matter of policy (6)
I believe the answer is:
insure
'you do' is the definition.
Although both the answer and definition are verbs in their base form, I can't see how they can define each other.
'as a matter of policy' is the wordplay.
'as' becomes 'in' (I've seen this in other clues).
'a matter of policy' becomes 'sure' (I am not sure about this - if you are sure you should believe this answer much more).
'in'+'sure'='INSURE'
'so' is the link.
Can you help me to learn more?
(Other definitions for insure that I've seen before include "Take out a policy" , "Rue sin (anag)" , "Provide protection against risk or damage" , "Guarantee against loss" , "Arrange for payment in the event of loss" .)