Upper limit, this person is saying (5)
I believe the answer is:
maxim
'saying' is the definition.
(maxim is a kind of saying)
'upper limit this person is' is the wordplay.
'upper limit' becomes 'max' (I can't explain this - if you can you should give a lot more credence to this answer).
'this person is' becomes 'I'm' ('this person' can refer to the person speaking).
'max'+'im'='MAXIM'
Can you help me to learn more?
(Other definitions for maxim that I've seen before include "- Gorky, Russian writer" , "Short pithy saying" , "Saw - type of gun" , "Short pithy statement" , "Saying - inventor of machine gun" .)