The most I'm saying! (5)
I believe the answer is:
maxim
'saying' is the definition.
(I know that maxim is a type of saying)
'the most i'm' is the wordplay.
'the most' becomes 'max' (maximum).
'max'+'im'='MAXIM'
(Other definitions for maxim that I've seen before include "Principle-first machine gun" , "- Gorky, Russian writer" , "Old machine-gun" , "Pithy statement of a general truth" , "General rule" .)