Can you Not So Fast a Defy?

No. You cannot use Not So Fast to counter Defy.

Not So Fast can only counter an enemy spell or ability that "chooses a friendly unit or gear." Defy does not choose a unit or gear — it targets a spell on the chain (one costing ≤4 energy and ≤1 power). Spells are not units or gear, so Defy doesn't meet Not So Fast's targeting requirement.

This is the same reason you can't Not So Fast another Not So Fast (RiftJudge FAQ #9272) — counter spells that target other chain items simply aren't valid targets for Not So Fast.