Wizkids Rules!

Nanclus says that the target ship does not roll defence dice this round. Does that mean that cards that say "When Defending" would not be usable, similarly to cloaked mines.

Cloaked Mines is a Combat Effect but players do not roll defense dice against it because it is not an attack. When Defending only applies to when your ship is being attacked. Nanclus does not prevent a ship from defending only from rolling defense dice.