Clue: Fractions of number in choppers to put on uniform on board (12)
Answer: SEVENTEENTHS
Explanation: Definition: Fractions
An envelope ('on board' as often is used to indicate IN – the envelope indicator – and SS, the outer part of the envelope)
 of EVEN ('uniform') plus TEENTH, another envelope (in') of N ('number') in TEETH ('choppers') – with 'to put on' indicating the order of the particles – all in SS (see above).