Adult in Slough ruined stew (7)
I believe the answer is:
goulash
'stew' is the definition.
(I know this)
'adult in slough ruined' is the wordplay.
'adult' becomes 'a' (abbreviation).
'in' means one lot of letters goes inside another.
'ruined' indicates anagramming the letters.
'slough' anagrammed gives 'goulsh'.
'a' placed within 'goulsh' is 'GOULASH'.
(Other definitions for goulash that I've seen before include "Hungarian meat and vegetable stew, with paprika" , "food" , "Stew of meat and veg, seasoned with paprika" , "Hungarian dish" , "Spicy Hungarian stew" .)