Butterfly in seawater? Most unusual (9)
I believe the answer is:
brimstone
'butterfly' is the definition.
(I've seen this before)
'in seawater? most unusual' is the wordplay.
'in' indicates putting letters inside.
'seawater?' becomes 'brine' ('brine' can be a synonym of 'seawater').
'unusual' is an anagram indicator.
'most' anagrammed gives 'msto'.
'brine' going around 'msto' is 'BRIMSTONE'.
(Other definitions for brimstone that I've seen before include "It robs men (anag) - old word for sulphur" , "Old name for sulphur" , "Fire partner" , "Yellow butterfly" , "Biblical sulphur that rained with fire on evil cities" .)