Clue: Battle nerve is to bandage cut in retreat (9)
Answer: GALLIPOLI
Explanation: Definition: Battle
An envelope ('to bandage') of POL, a reversal ('in retreat') of LOP ('cut') in GALL ('nerve') plus II ('Is', as a jocular plural of i). As the invasion of Gallipoli in the First World War led to eight months of fighting, it is more often called a campaign, rather than a battle.