Miracle followed about a key (4)
I believe the answer is:
clef
'a key' is the definition.
(I have seen 'Musical key' mean 'clef' so perhaps 'key' could also mean 'clef')
'miracle followed about' is the wordplay.
'about' indicates a hidden word (the clue goes around or about the answer).
'CLEF' is hidden in the letters of 'miracle followed'.
(Other definitions for clef that I've seen before include "Musical stave sign" , "Symbol on stave in musical score" , "Symbol on a stave indicating pitch" , "Symbol written on sheet music to indicate pitch" , "Mark determining musical pitch of notes" .)