### Clue: Bottom gear for some kids on bikes? (5,8)
### Answer: SHORT TROUSERS 
 ### Explanation: Definition: Bottom gear for some kids
 Cryptic defn: "trousers"(clothes;gear covering the bottom), and those worn by kids are of a short length. The "bottom gear … on bikes" is a misdirection to the lowest gearing on a bicycle, used when cycling up steep gradients. I wonder if there is something more, as this doesn't seem to be sufficiently cryptic.