If the clan has been official, you can't just take the tag unless you revive it, which involves gathering 3 former members of the clan. The following applies for taking the tag of an unofficial clan:
Originally Posted by
Clan Rules & Guidelines
your clan will be considered 'dead' and lose its rights to its current name and tag if there hasn't been a post in either your discussion or recruitment threads by a clan member in over a year. If your clan also has under five individual members (not alts), this deadline is 6 months instead.
So basically wait 6 months or a year and the tag can be yours if the tag you wish to take is being used by a dead unofficial clan.