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 in another clue)
'sure to be wrong about a teetotaller' is the wordplay.
'to be wrong' indicates anagramming the letters.
'about' indicates putting letters inside.
'a teetotaller' becomes 'tt' (abbreviation).
'sure' is an anagram of 'uers'.
'uers' placed around 'tt' is 'UTTERS'.
'what's' is the link.
(Other definitions for utters that I've seen before include "Speaks, says" , "Publishes" , "Speaks, voices" , "Speaks, articulates" , "Expresses audibly" .)