Tendency toward chaos (7)
I believe the answer is:
entropy
'tendency toward chaos' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for entropy that I've seen before include "Physical quantity linked to disorder" , "Physical state" , "Term in physics" , "Degree of disorder (physics)" , "Tory pen (anag) - measure of disorder" .)