Grumble has come from middle of stomach, say (6)
I believe the answer is:
mutter
'grumble' is the definition.
(I know that grumble can be written as mutter)
'middle of stomach say' is the wordplay.
'middle of' indicates the central letters.
'say' becomes 'utter' (to utter something is to speak or say it).
The middle of 'stomach' is 'm'.
'm'+'utter'='MUTTER'
'has come from' acts as a link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for mutter that I've seen before include "Lack of energy" , "Speak quietly and indistinctly" , "Talk under one's breath" , "Grumble in secret; mother (German)" , "Speak or grumble in low clear voice" .)