Foil wrapper for sweet that is filled with the reverse of goodness! (6)
I believe the answer is:
stymie
'foil' is the definition.
(thesaurus)
'wrapper for sweet that is filled with the reverse of goodness' is the wordplay.
'wrapper for' says to hollow out the word (remove centre letters).
'that is' becomes 'ie' ('i.e.' can be a synonym of 'that is').
'filled with' indicates putting letters inside.
'the reverse of' shows that the letters should be reversed in order.
'goodness' becomes 'my' (both are exclamations of surprise).
'sweet' with its centre taken out is 'st'.
'my' in reverse letter order is 'ym'.
'st'+'ie'='stie'
'stie' enclosing 'ym' is 'STYMIE'.
(Other definitions for stymie that I've seen before include "Snooker" , "Obstruction (originally at golf)" , "Thwart, foil" , "Thwart (as once was possible on the green)" , "Hinder the progress of (something)" .)