Clue: Spreading dirt around hospital is what 2 need to become 12 (5)
Answer: THIRD
Explanation: Spreading dirt around hospital is what 2 need to become 12 (5)
***(dirt)** [anag:*spreading*] *around* **H** (hospital) 
*2 would need a THIRD to become a "threesome" (the answer to 12 across)