Very quiet religious mission sent out to cover America (10)
I believe the answer is:
pianissimo
'very quiet' is the definition.
(I've seen this before)
'religious mission sent out to cover america' is the wordplay.
I cannot quite understand how this works, but
'religious' could be 'pi' (british abbreviation for pious) and 'pi' is present in the answer.
an anagram of 'mission' is 'nissimo' which is present in the answer.
'america' could be 'a' (common abbreviation eg in USA) and 'a' is present in the answer.
This accounts for all the letters.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for pianissimo that I've seen before include "Very softly" , "Very quiet" , "Very quietly (music)" , "Very soft (music)" , "(To be played) very quietly" .)