Chaotic parts may be disastrous (12)
I believe the answer is:
catastrophic
'disastrous' is the definition.
(I've seen this before)
'chaotic parts may be' is the wordplay.
'may be' is an anagram indicator.
'chaotic'+'parts'='chaoticparts'
'chaoticparts' with letters rearranged gives 'CATASTROPHIC'.
(Other definitions for catastrophic that I've seen before include "Causing sudden damage" , "Disastrous" , "Extremely unfortunate" , "Chaotic parts (anag)" .)