Simple melody stirred Vatican guard's heart (8)
I believe the answer is:
cavatina
'simple melody' is the definition.
(I've seen this in another clue)
'stirred vatican guard's heart' is the wordplay.
'stirred' indicates anagramming the letters.
'heart' says to take the centre.
The middle letter of 'guard' is 'a'.
'vatican' with letters rearranged gives 'cavatin'.
'cavatin'+'a'='CAVATINA'
(Other definitions for cavatina that I've seen before include "simple song" , "Simple operatic aria" , "air" .)