Not having the front seat on a tandem's rotten (5)
I believe the answer is:
addle
'rotten' is the definition.
(I've seen this before)
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for addle that I've seen before include "Get rotten" , "(Of eggs) become rotten" , "Muddle or confuse" , "Make rotten" , "Make confused - go bad" .)