Mine, I agree, finally hit bottom (9)
I believe the answer is:
fundament
'bottom' is the definition.
(I've seen this before)
'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 last letter of 'hit' is 't'.
'fund'+'amen'+'t'='FUNDAMENT'
(Other definitions for fundament that I've seen before include "bottom" , "Underlying basis - foundation" , "seat" .)