Ahh ok, well you don't have to be a Department head, and of course Tactical/Security head is taken. You could still be in the department though, maybe Assisstant Chief Tactical/Security Officer then?
In a combat situation you take over if the Chief is out of action on the bridge. Other than that your generally organising the security forces in the event of possible boarding operations and oversee-ing the distribution of weaponry. In a non-combat situation you take the Tactical/Security console on the bridge when the Chief isn't on his shift.
One of the most complete and easy to use resources for most things Trek I've found is this:
http://database.obsidianfleet.net/wiki/index.php/Main_PageWhile there are parts specific to their fleet in there the general department descriptions and position descriptions are pretty good.