Brawler talent tree

Counterpunch

Reference Book: Star Wars Saga Edition Scum and Villainy

When you Fight Defensively, any adjacent creature that attacks you provokes an Attack of Opportunity from you.

Defensive Acuity also buffs attacks when you fight defensively, but it's from the Jedi class. No other prereqs though

The Shockboxer Talent Tree has something useful for you if you're fighting Unarmed (Your GM will have to determine if Fist + Riot Shield qualifies)

Defensive Jab

Prerequisite: Retaliation Jab

When you are Unarmed and take the Fight Defensively Action, you can make a single Unarmed attack as a Free Action against an adjacent target.

Also, you can get some other defensive perks for Fighting Defensively with the Wary Defender (+2 competence bonus to Fortitude and Will Defenses while Fighting Defensively) and Resolute Stance (+2/+5 morale bonus to Will Defense when Fighting Defensively) feats.