Reasoning
Warcraft Tavern's Priest guide recommends Tailoring because Priests wear cloth, and Forever adds gear oriented around specific damage/healing schools plus new level-10 Spell Power belts. Alchemy is the paired endgame profession because Priests are heavily potion-reliant and Forever adds many new buff and damage/crowd-control consumables; while leveling the guide suggests pairing Tailoring with a gatherer like Herbalism (or Alchemy with Mining). A secondary, unverified source instead pairs Tailoring with Enchanting for Priests, so the crafting profession (Tailoring) is well-agreed but the companion profession is disputed between Alchemy and Enchanting. Source picks: Warcraft Tavern Priest guide: Tailoring + Alchemy; Koroboost (unverified): Tailoring + Enchanting.
Confidence note
Analysis built from sourced profession facts