Muttered that mother was very sorry (7)
I believe the answer is:
mumbled
'muttered' is the definition.
('mumble' can be a synonym of 'mutter')
'mother was very sorry' is the wordplay.
I cannot really understand how this works, but
'mother' could be 'mum' (informal term for mother) and 'mum' is found within the answer.
The remaining letters 'bled' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
'that' is the link.
Can you help me to learn more?
(Another definition for mumbled that I've seen is " Muttered".)