Peter and Liz I left cooking biscuit (7)
I believe the answer is:
pretzel
'biscuit' is the definition.
(thesaurus)
'peter and liz i left cooking' is the wordplay.
'and' says to put letters next to each other (I've seen this in other clues).
'left' suggests deleting specific letters.
'cooking' indicates an anagram.
'peter' after 'liz' is 'lizpeter'.
'lizpeter' with 'i' taken out is 'lzpeter'.
'lzpeter' with letters rearranged gives 'PRETZEL'.
(Other definitions for pretzel that I've seen before include "Curly savoury biscuit" , "Salted biscuit, shaped like a knot" , "Twisted biscuit" , "Knotted crisp biscuit" , "Knot-shaped cracker" .)