One's admiral slain finally at sea (6)
I believe the answer is:
nelson
'one's admiral slain finally at sea' is the definition.
I know that 'admiral' relates to this answer.
'one's admiral slain finally at sea' is the wordplay.
'finally' says to take the final letters.
'at sea' indicates an anagram.
The final letters of 'admiral slain' are 'ln'.
'ones'+'ln'='onesln'
'onesln' is an anagram of 'NELSON'.
(Other definitions for nelson that I've seen before include "One-eyed English admiral seen on pillar" , "Admiral called 5" , "Sea-lord who loved Lady Hamilton" , "Horatio ..., British admiral" , "Victorious flag officer" .)