Sulphur most volatile in seawater (9)
I believe the answer is:
brimstone
'sulphur' is the definition.
(I've seen this before)
'most volatile in seawater' is the wordplay.
'volatile' indicates anagramming the letters.
'in' means one lot of letters goes inside another.
'seawater' becomes 'brine' (synonyms).
'most' with letters rearranged gives 'msto'.
'msto' put inside 'brine' is 'BRIMSTONE'.
(Other definitions for brimstone that I've seen before include "Sulphur (when associated with hellfire)" , "It robs men (anag) - old word for sulphur" , "Fire partner" , "Yellow butterfly" , "Old name for sulphur" .)