One of two I plunged into volatile liquid (6)
I believe the answer is:
either
'one of two' is the definition.
(I've seen this before)
'i plunged into volatile liquid' is the wordplay.
'plunged into' indicates putting letters inside.
'volatile liquid' becomes 'ether' (I've seen this before).
'i' inserted inside 'ether' is 'EITHER'.
(Other definitions for either that I've seen before include "could be one of two" , "Choice of two" , "No matter which (of two)" , "Each of two; moreover" , "Word indicating the following alternatives" .)