Promise made by page left on margin (6)
I believe the answer is:
pledge
'promise' is the definition.
(I know that pledge has the is a more specific form of the action promise)
'page left on margin' is the wordplay.
'page' becomes 'p'.
'left' becomes 'l' (common abbreviation).
'on' is a charade indicator (letters next to each other).
'margin' becomes 'edge' (synonyms).
'p'+'l'+'edge'='PLEDGE'
'made by' acts as a link.
(Other definitions for pledge that I've seen before include "Solemn promise" , "Give one's word" , "Promise, word of honour" , "Solemn undertaking" , "make toast" .)