Treaty port nearly covering up chaos (7)
I believe the answer is:
entropy
'chaos' is the definition.
(I've seen this in another clue)
'treaty port nearly covering up' is the wordplay.
'covering' indicates the answer is hidden within the clue.
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'yportne' is hidden within 'treaty port nearly'.
'yportne' backwards is 'ENTROPY'.
(Other definitions for entropy that I've seen before include "Physical quantity linked to disorder" , "Try open (anag) - disorder in the system" , "state of increasing disorder" , "Degree of disorder (physics)" , "Measure of the disorder of a system" .)