Promise place on Fringe (6)
I believe the answer is:
Here is my best explanation:
'promise' is the definition.
(pledge is a kind of promise)
'place on fringe' is the subsidiary indication.
'place' becomes 'pl' (I've seen this in other clues).
'on' means one lot of letters go next to another.
'fringe' becomes 'edge' ('edge' can be a synonym of 'fringe').
(Other definitions for pledge that I've seen before include "Promise solemnly and formally", "Promise or pawn", "Promise - security", "Undertaking", "Deposit", "Vow; security", "Plight", "Vow, affirm", "Solemn undertaking" and "Covenant".)