Pledged to have shelf (5)
I believe the answer is:
ledge
'shelf' is the definition.
(synonyms)
'pledged to have' is the wordplay.
'to have' says the answer is hidden in the clue.
'LEDGE' is hidden in the letters of 'pledged'.
(Other definitions for ledge that I've seen before include "Sill" , "Mantelpiece" , "Shelf-life projection" , "Ridge or shelf" , "Projecting ridge on a mountain" .)