... mass being sung at the correct pitch (6)
I believe the answer is:
monkey
'mass being sung at the correct pitch' is the definition.
I can't tell whether this defines the answer.
'mass being sung at the correct pitch' is the wordplay.
'mass' becomes 'm' (symbol used in Physics e.g. f=ma).
'being sung at the correct pitch' becomes 'on key' (on the right musical key).
'm'+'onkey'='MONKEY'
(Other definitions for monkey that I've seen before include "Primate with a long tail" , "Naughty child - 500 pounds" , "Small primate" , "Naughty child; primate" , "PS500 (sl.)" .)