Gunners unfurled flag held by sailor in battle (9)
I believe the answer is:
trafalgar
'battle' is the definition.
(Trafalgar is an example)
'gunners unfurled flag held by sailor' is the wordplay.
'gunners' becomes 'ra' (Royal Artillery).
'unfurled' indicates anagramming the letters.
'held by' indicates putting letters inside.
'sailor' becomes 'tar' (tar is a kind of sailor**).
'flag' is an anagram of 'falg'.
'ra'+'falg'='rafalg'
'rafalg' put inside 'tar' is 'TRAFALGAR'.
'in' acts as a link.
(Other definitions for trafalgar that I've seen before include "London Square with Nelson column" , "1805 sea battle" , "Nelson's eighteen hundred and five victory" , "British naval victory" , "19th century battle" .)