Says what's sure to be wrong about a teetotaller (6)
I believe the answer is:
utters
'says' is the definition.
(I've seen this before)
'sure to be wrong about a teetotaller' is the wordplay.
'to be wrong' is an anagram indicator (I've seen 'is wrong' mean this).
'about' is an insertion indicator.
'a teetotaller' becomes 'tt' (abbreviation).
'sure' anagrammed gives 'uers'.
'uers' going around 'tt' is 'UTTERS'.
'what's' acts as a link.
(Other definitions for utters that I've seen before include "Speaks, says" , "Publishes" , "Speaks, voices" , "Speaks, articulates" , "Expresses audibly" .)