There is a dent in the store sign because some drunk guy punched it.
A watch over board is a board that goes over your store and watches over everything. What board on a store watches everything? It's the one that gets the most visibility, so it's probably the store sign.
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well. Be careful of the はん changing to ばん.