The incident occurred at The Dolphin and Anchor, in West Street, on Saturday evening (15 January).
An altercation occurred, which resulted in a 20-year-old local man sustaining injuries including a broken ankle.
Police are now appealing for witnesses to come forward with any information.
It is unknown exactly what time the assault occurred, but the pub was busy with customers that evening and officers and asking anyone with information to report it online or call 101, quoting serial 864 of 16/01.
You can also contact the independent charity Crimestoppers anonymously on 0800 555 111 or report it online.