Lowest note? Second in sung Mass (7)
I believe the answer is:
minimum
'lowest' is the definition.
(similar in meaning)
'note? second in sung mass' is the wordplay.
'note?' becomes 'minim' (musical note).
'second in sung' becomes 'u' (2nd letter of 'sung').
'mass' becomes 'm' (symbol used in Physics e.g. f=ma).
'minim'+'u'+'m'='MINIMUM'
(Other definitions for minimum that I've seen before include "Lowest value" , "Lower limit" , "Bottom" , "Lowest point" , "Smallest possible quantity" .)