Peak journey across capital of Germany (5)
I believe the answer is:
ridge
'peak' is the definition.
(I've seen this before)
'journey across capital of germany' is the wordplay.
'journey' becomes 'ride' (riding is a kind of journeying**).
'across' is an insertion indicator.
'capital of' indicates taking the first letters.
The first letter of 'germany' is 'g'.
'ride' placed around 'g' is 'RIDGE'.
(Other definitions for ridge that I've seen before include "Long, narrow range of hills" , "Line of raised ground" , "Raised crease" , "Raised earth between furrows" , "High ground" .)