One sudden movement leads to accidental cloudiness in decidedly pungent chemical (6,4)
I believe the answer is:
acetic acid
'pungent chemical' is the definition.
(smelly chemical)
'one sudden movement leads to accidental cloudiness in decidedly' is the wordplay.
'one' becomes 'ace' (ace has a value of one in some card games).
'sudden movement' becomes 'tic' (an involuntary physical movement).
'leads to' suggests taking the first letters.
The first letters of 'accidental cloudiness in decidedly' is 'acid'.
'ace'+'tic'+'acid'='ACETIC ACID'
(Other definitions for acetic acid that I've seen before include "pickling stuff?" , "Component of vinegar" .)