Chemical partly unstable, a chloride (6)
I believe the answer is:
bleach
'chemical' is the definition.
(I've seen this before)
'partly unstable a chloride' is the wordplay.
'partly' indicates a hidden word (part of the clue is used).
'BLEACH' is hidden in the letters of 'unstable a chloride'.
(Other definitions for bleach that I've seen before include "Go white" , "Disinfectant and whitener" , "Blanche" , "Whiten with a belch" , "Make white by sun or chemicals" .)