Seemed upset with underpowered bike (5)
I believe the answer is:
moped
'bike' is the definition.
(I know that moped is a type of minibike)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for moped that I've seen before include "means of transport" , "dejected" , "Despaired" , "simple conveyance" , "Power-assisted vehicle" .)