Does Rek'Sai Breacher work with Shadow Clone?

Yes — Rek'Sai, Breacher lets your Shadow Clone tokens enter ready, but only if you pay the Accelerate cost.

Shadow Clones (e.g., from Death Mark or Zed, From the Shadows) are played from a zone other than a player's hand, so Rek'Sai grants them Accelerate as they're being played. Accelerate is an optional additional cost — 1 Energy and 1 Shadow Power — paid during the Pay Costs step of playing the token. Since the Shadow Clone has a single domain ([S]), the Power portion must be Shadow Power, not generic [A]. If you don't pay, it enters exhausted as normal. (FAQ #11708, Rule 805.1.a.1)