Graduate, falling short and beginning to stagnate, grumbles (8)

'grumbles' is the definition.
The answer and definition can be both to do with communicating as well as being verbs in their -s form.
Perhaps there's a link between them I don't understand?

'graduate falling short and beginning to stagnate' is the wordplay.
'graduate' becomes 'MA' (abbreviation for Master of Arts).
'falling short' becomes 'under'.
'and' means one lot of letters go next to another.
'beginning to' says to take the initial letters.
The initial letter of 'stagnate' is 's'.
'ma'+'under'+'s'='MAUNDERS'

Can you help me to learn more?