As per title, would fearlessness add a hit result (assuming range 1 and other requirements) to each roll for clusters missiles or would it just be once for the entire attack?

Unless they're limited in some other way, "When Attacking" and "When Defending" effects have two distinct activation windows during a "Perform This Attack Twice" attack like Cluster Missiles. So they can activate twice.

Righto, wasn't sure if this fell under the Clusters being one technical attack as per the FCS interaction with them.

Righto, wasn't sure if this fell under the Clusters being one technical attack as per the FCS interaction with them.

They're still two attacks, they just only trigger "When You Declare a Target", "After Attacking", and "After Defending" effects once.

Righto, wasn't sure if this fell under the Clusters being one technical attack as per the FCS interaction with them.

Clusters is not one attack. It's two attacks, with "after attacking/defender" effects skipped after the first one, and "declare target" skipped at the start of the second one. Thinking of it as "one attack" leads to all kinds of wrong interpretations.

Anything that says "When attacking" or "when defending" still triggers on both attacks.

Righto, wasn't sure if this fell under the Clusters being one technical attack as per the FCS interaction with them.

Cluster Missiles and Twin Laser Turrets are two attacks and Fire Control System triggers after both attacks have been resolved at step 8 (see page 6 of the FAQ).

The sequence for Cluster Missiles or Twin Laser Turrets is normally this:

Step 1 - Declare Target

Step 2-3 - Roll and Modify Attack Dice

Step 4-5 - Roll and Modify Defense Dice

Step 6 - Compare Results

Step 7 - Deal damage (if you hit)

and once again for the second attack...

Step 2-3 - Roll and Modify Attack Dice

Step 4-5 - Roll and Modify Defense Dice

Step 6 - Compare Results

Step 7 - Deal damage (if you hit)

As EdgeOfDreams said, don't think of Cluster Missiles or Twin Laser Turrets as one "technical" attack, because that thinking will steer you into all sorts of wrong conclusions.