Is contingent on editor reviewing page margins (7)
I believe the answer is:
depends
'is contingent' is the definition.
(I've seen this in another clue)
'editor reviewing page margins' is the wordplay.
'editor' becomes 'ed' (abbreviation).
'reviewing' is a reversal indicator (I've seen 'review' mean this).
'page' becomes 'p'.
'margins' becomes 'ends' (end can mean an edge or margin).
'ed' written backwards gives 'de'.
'de'+'p'+'ends'='DEPENDS'
'on' acts as a link.
(Other definitions for depends that I've seen before include "Counts (on)" , "Relies for support" , "Is contingent upon" , "Relies or hangs on" .)