Opening of a key appeal (4)
I believe the answer is:
adit
'opening' is the definition.
(thesaurus)
'a key appeal' is the wordplay.
'key' becomes 'd' (I've seen this in other clues).
'appeal' becomes 'it'.
'a'+'d'+'it'='ADIT'
'of' is the link.
(Other definitions for adit that I've seen before include "Da, it is a mine entrance" , "Mine-shaft" , "Almost horizontal mine shaft" , "Mine opening" , "Horiz. mine entrance" .)