Teenager astride exercise bike (5)
I believe the answer is:
moped
'bike' is the definition.
(moped is a kind of bike)
'teenager astride exercise' is the wordplay.
'teenager' becomes 'mod' (mod is a kind of teenager).
'astride' indicates putting letters inside.
'exercise' becomes 'pe' (Physical Education).
'mod' going around 'pe' is 'MOPED'.
(Other definitions for moped that I've seen before include "Light motor cycle" , "Despaired" , "means of transport" , "Vehicle - was blue" , "Two-wheeled transport -- was in low spirits" .)