Bear with Set's Mechanic is not legal in any format.
As your first combat phase ends during your turn, if one or more creatures you control have aggressive, you have another combat phase before your second main phase. This is called the aggressive combat phase.
Creatures without aggressive can't attack during an aggressive combat phase.
If you control no creatures with aggressive as your first combat phase ends, but control one later during your turn, you won't have an aggressive combat phase.