Mine, I agree, finally hit bottom (9)
I believe the answer is:
Here is my best explanation:
'bottom' is the definition.
('fundament' can be a synonym of 'bottom')
'mine i agree finally hit' is the wordplay.
'mine' becomes 'fund' (both can mean a source of something).
'i agree' becomes 'amen' (I've seen this before).
'finally' indicates one should take the final letters.
The final letter of 'hit' is 't'.
(Other definitions for fundament that I've seen before include "bottom", "Underlying basis - foundation" and "seat".)