Sat stirring in basis for cheese sauce (7)
I believe the answer is:
custard
'sauce' is the definition.
(I've seen this before)
'sat stirring in basis for cheese' is the wordplay.
'stirring' indicates anagramming the letters.
'in' is an insertion indicator.
'basis for cheese' becomes 'curd' (I've seen this before).
'sat' is an anagram of 'sta'.
'sta' going within 'curd' is 'CUSTARD'.
(Other definitions for custard that I've seen before include "-- Pie; -- apple" , "Cooked mixture of milk and eggs" , "Pudding accompaniment" , "Pudding sauce" , "Dessert sauce" .)