This is all I'm going to say on this and you can take it or leave it? (9)
I believe the answer is:
ultimatum
'this is all' is the definition.
The definition suggests a singular noun which matches the answer.
'i'm going to say on this and you can take it or leave it?' is the wordplay.
I cannot really see how this works, but
'i'm' could be 'im' and 'im' is found in the answer.
'you' could be 'u' (I've seen this in other clues) and 'u' is located in the answer.
'it' could be 't' (abbreviation. e.g. in 'tis) and 't' is present in the answer.
'it?' could be 't' (abbreviation. e.g. in 'tis) and 't' is found in the answer.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for ultimatum that I've seen before include "Threat" , "Final demand or statement of terms" , "Final point" , "Final proposition or demand" , "Uncompromising demand" .)