Found in bistro, a stupendous cook (5)
I believe the answer is:
roast
'cook' is the definition.
(roasting is a kind of cooking)
'found in bistro a stupendous' is the wordplay.
'found in' indicates the answer is hidden within the clue.
'ROAST' is hidden in the letters of 'bistro a stupendous'.
(Other definitions for roast that I've seen before include "Criticise severely" , "Swelter" , "Traditional Sunday joint?" , "cooked a certain way" , "Cook in oven or by exposure to heat" .)