Teetotaller left cheese out that's crumbly (7)
I believe the answer is:
'crumbly' is the definition.
'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.
'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 confectionery made with nuts", "Easy to break", "Easily cracked or snapped", "Type of toffee made with nuts".)