Clue: Fractions of number in choppers to put on uniform on board (12)
Answer: SEVENTEENTHS
Explanation: Fractions of number in choppers to put on uniform on board (12)
**N**(umber) *in* **TEETH** ("choppers") *put on* **EVEN** ("uniform") *in* **SS** ("on board", *ie on a steamship*), *so* **S-EVEN-TEE(N)TH-S