Battle flag a tar at sea raised originally (9)
I believe the answer is:
trafalgar
'battle' is the definition.
(Trafalgar is an example)
'flag a tar at sea raised originally' is the wordplay.
'at sea' is an anagram indicator.
'originally' suggests taking the first letters.
The initial letter of 'raised' is 'r'.
'flag'+'a'+'tar'='flagatar'
'flagatar' anagrammed gives 'trafalga'.
'trafalga'+'r'='TRAFALGAR'
(Other definitions for trafalgar that I've seen before include "1805 sea battle" , "Cape" , "Public space in central London" , "British naval victory" , "Victory of 1805" .)