Duke and king behind plan (5)
I believe the answer is:
draft
'plan' is the definition.
(I know that draft is a more specific form of plan)
'duke and king behind' is the wordplay.
'duke' becomes 'd' (abbreviation).
'and' means one lot of letters go next to another.
'king' becomes 'R' (abbreviation of Latin rex).
'behind' becomes 'aft' (aft means towards or at the end of a ship).
'd'+'r'+'aft'='DRAFT'
(Other definitions for draft that I've seen before include "book's outline" , "plan outlined" , "conscript" , "Provisional document" , "form of conscription" .)