Good journey taken round long hilltop (5)
I believe the answer is:
ridge
'long hilltop' is the definition.
(I've seen this before)
'good journey taken round' is the wordplay.
'good' becomes 'g' (abbreviation).
'journey' becomes 'ride' (riding is a kind of journeying).
'taken round' means one lot of letters goes inside another.
'g' put inside 'ride' is 'RIDGE'.
(Other definitions for ridge that I've seen before include "Top of a long narrow hill" , "Peak" , "Part of mountain" , "Raised strip between two furrows" , "High land" .)