Cook in bistro astounds them (5)
I believe the answer is:
roast
'cook' is the definition.
(I know that roast is a more specific form of cook)
'in bistro astounds them' is the wordplay.
'in' indicates the answer is hidden within the clue.
'ROAST' is hidden in the letters of 'bistro astounds them'.
(Other definitions for roast that I've seen before include "Get very warm" , "open-air party in US" , "Meat to get hot" , "Have a go at" , "Become very warm" .)