Compactness altered destiny (7)
I believe the answer is:
density
'compactness' is the definition.
(I know that density is a type of compactness)
'altered destiny' is the wordplay.
'altered' indicates anagramming the letters.
'destiny' anagrammed gives 'DENSITY'.
(Other definitions for density that I've seen before include "It's usually measured in grammes per cc" , "The compactness of a substance" , "Degree of compactness" , "Concentrated condition as of fog" , "Closeness" .)