If a creature is returned to hand during the declare attackers step does combat end?

Returning a creature to hand during the declare attackers step does not end combat or skip any remaining combat steps. The combat phase proceeds through its full sequence: beginning of combat, declare attackers, declare blockers, combat damage, and end of combat (CR 506.1).

If the creature that was returned to hand was an attacking creature, it simply leaves the combat. It is no longer an attacking creature, and the attack it represented is removed. The remaining attackers (if any) continue as normal. If there are no other attackers, combat still continues through its steps — they just resolve with no attackers present (CR 506.4).

If the returned creature was not yet declared as an attacker (since we are still in the declare attackers step), it simply ceases to be a legal attacker. The active player finishes declaring attackers without it.

Concrete example: You attack with a 3/3 and a 2/2. In response to nothing specific, your opponent plays Unsummon targeting your 3/3 during the declare attackers step. The 3/3 returns to your hand and leaves combat. Your 2/2 remains as the sole attacker, and play proceeds normally to the declare blockers step.

The key rule is CR 506.4: "A creature that is removed from combat is no longer an attacking or blocking creature. It retains no memory of being in combat." Nothing in the rules causes the combat phase to end simply because a creature leaves combat or the battlefield.