Ace in Slough cooked stew (7)
I believe the answer is:
goulash
'stew' is the definition.
(I know that goulash is a type of stew)
'ace in slough cooked' is the wordplay.
'ace' becomes 'a' (abbreviation).
'in' indicates putting letters inside.
'cooked' indicates an anagram (letters cooked into a new form).
'slough' anagrammed gives 'goulsh'.
'a' put within 'goulsh' is 'GOULASH'.
(Other definitions for goulash that I've seen before include "deal that nobody likes" , "Spicy Hungarian stew" , "Stew flavoured with paprika" , "Stew of meat and veg, seasoned with paprika" , "Rich stew of Hungarian origin" .)