Breaking law, say, that describes how things fall down (6)
I believe the answer is:
always
'that describes how things fall down' is the definition.
('what goes up must come down')
'breaking law say' is the wordplay.
'breaking' is an anagram indicator.
'law'+'say'='lawsay'
'lawsay' anagrammed gives 'ALWAYS'.
(Other definitions for always that I've seen before include "Eternally" , "For good" , "whatever the circumstances" , "For ever" , "Perpetually" .)