Very quiet and holy mission a disaster (10)
I believe the answer is:
pianissimo
'very quiet' is the definition.
(I've seen this before)
'holy mission a disaster' is the wordplay.
'holy' becomes 'pi' (I've seen this in other clues).
'disaster' is an anagram indicator.
'mission'+'a'='missiona'
'missiona' is an anagram of 'anissimo'.
'pi'+'anissimo'='PIANISSIMO'
'and' acts as a link.
(Other definitions for pianissimo that I've seen before include "Very quietly (music)" , "Very soft (music)" , "Very softly" , "Very quiet" , "(To be played) very quietly" .)