Extremely dire, lengthy journey, not a direct route (6)
I believe the answer is:
detour
'not a direct route' is the definition.
(a detour is an indirect route)
'extremely dire lengthy journey' is the wordplay.
'extremely' means to remove the middle letters (only the extremes of the word are used).
'lengthy journey' becomes 'tour' (a journey taking in multiple destinations).
'dire' with its centre removed is 'de'.
'de'+'tour'='DETOUR'
(Other definitions for detour that I've seen before include "Deviation from direct course" , "Deviate from planned route, go around" , "Circuitous route" , "Deviation from usual course" , "Temporary roundabout way" .)