Say something that's extreme (5)
I believe the answer is:
utter
'extreme' is the definition.
(I've seen this before)
'say something' is the wordplay.
I cannot really understand how this works, but
'something' could be 'er' (I've seen this in other clues) and 'er' is present in the answer.
This may be the basis of clue (or it may be nonsense).
'that's' is the link.
Can you help me to learn more?
(Other definitions for utter that I've seen before include "(Of counterfeit money) put into circulation - say" , "Speak; complete" , "Make sounds (but not necessarily words)" , "Sheer" , "outright" .)