Changeable, volatile (9)

I believe the answer is:

mercurial

I believe this clue is a double definition.

'changeable' is the first definition.
(similar in meaning)

'volatile' is the second definition.
(I've seen this before)

(Other definitions for mercurial that I've seen before include "Changing unpredictably" , "Volatile" , "Temperamental" , "Prone to sudden mood swings" , "Sprightly" .)

I've seen this clue in The Times.

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