Element oxygen many initially discovered in sea water (7)
I believe the answer is:
bromine
'element' is the definition.
(bromine is a kind of element)
'oxygen many initially discovered in sea water' is the wordplay.
'initially' indicates taking the first letters.
'discovered in' is an insertion indicator.
'sea water' becomes 'brine' (I've seen this before).
The first letters of 'oxygen many' is 'om'.
'om' put within 'brine' is 'BROMINE'.
(Other definitions for bromine that I've seen before include "Red liquid element" , "Halogen element" , "Element Br" , "Dark red toxic element" .)