Miracle found to hold key (4)
I believe the answer is:
clef
'key' is the definition.
(I have seen 'Musical key' mean 'clef' so perhaps 'key' could also mean 'clef')
'miracle found to hold' is the wordplay.
'to hold' indicates a hidden word (I've seen 'holds' mean this).
'CLEF' can be found hidden inside 'miracle found'.
(Other definitions for clef that I've seen before include "Sign at start of stave" , "Symbol on a stave to show the pitch" , "Symbol on a stave indicating pitch" , "Musical notation on a staff" , "Treble or bass symbol?" .)