Plan to arrange copy to include 51 (6)
I believe the answer is:
policy
'plan' is the definition.
(I know that policy is a type of plan of action)
'arrange copy to include 51' is the wordplay.
'arrange' indicates an anagram.
'to include' is an insertion indicator.
'51' becomes 'LI' (Roman numerals).
'copy' is an anagram of 'pocy'.
'pocy' placed around 'li' is 'POLICY'.
'to' is the link.
(Other definitions for policy that I've seen before include "Adopted principle" , "(E.g. government) line" , "Principle of action by an organization" , "Plan of action of organisation - honesty is the best one" , "Plan of action - insurance document" .)