Teetotaller left cheese out that's crumbly (7)
I believe the answer is:
brittle
'crumbly' is the definition.
(thesaurus)
'teetotaller left cheese out' is the wordplay.
'teetotaller' becomes 'tt' (abbreviation).
'left' becomes 'l' (common abbreviation).
'cheese' becomes 'brie' (Brie is an example).
'out' means one lot of letters goes inside another.
'tt'+'l'='ttl'
'ttl' going into 'brie' is 'BRITTLE'.
'that's' is the link.
(Other definitions for brittle that I've seen before include "Liable to snap" , "Hard but easily broken" , "Hard but liable to break easily" , "Easily cracked or snapped" , "Hard confectionery made with nuts" .)