Challenge on reflection no good: how to go from London to Leeds? (6)
I believe the answer is:
impugn
'challenge' is the definition.
(impugning is a kind of challenging)
'reflection no good how to go from london to leeds?' is the wordplay.
'reflection' is a reversal indicator.
'no' becomes 'n' (common abbreviation).
'good' becomes 'g' (abbreviation).
'how to go from london to leeds?' becomes 'upmi' (resembles UP M1 - the M1 connects London to Leeds).
'n'+'g'+'upmi'='ngupmi'
'ngupmi' back-to-front is 'IMPUGN'.
'on' acts as a link.
(Other definitions for impugn that I've seen before include "Express doubts about truth or honesty of, say motives" , "Blacken the name of" , "Attack as false, challenge" , "Call into question; criticise" , "Nip mug badly and call in question" .)