Can I trigger Defender effect after Reaction?

It depends on what you mean by "after."

If you're asking about the normal Combat Showdown flow — no. "When I defend" triggers are placed on the chain before any player can play a Reaction. The order is fixed (Rule 459.2.d): Attack triggers → non-defender triggers → Defend triggers are placed last. Only after all triggers and their targets are locked in does the state become Closed and reactions become legal. You cannot play a Reaction first and then have a defender trigger appear in response to it during this initial chain (FAQ #10000, #11216).

However, if a Reaction causes a new unit to arrive at the battlefield mid-combat (e.g., Flash moving a unit in), that unit will gain the Defender designation during the cleanup after that action resolves (Rule 459.2.b.4.a). Since it's gaining Defender for the first time this combat, its "When I defend" ability will trigger — creating a new chain and a new reaction window (FAQ #4303, #1829). So yes, in that specific case, a defender trigger can happen after a Reaction resolves.