Speak indistinctly in mass, say (6)
I believe the answer is:
mutter
'speak indistinctly' is the definition.
(I've seen this before)
'mass say' is the wordplay.
'mass' becomes 'm' (symbol used in Physics e.g. f=ma).
'say' becomes 'utter' (I've seen this before).
'm'+'utter'='MUTTER'
'in' acts as a link.
(Other definitions for mutter that I've seen before include "Grumble under one's breath" , "Lack of energy" , "Barely audible utterance" , "Grumble in secret; mother (German)" , "German violinist" .)