Enter the capital of Germany when travelling (5)
I believe the answer is:
going
'travelling' is the definition.
('go' can be a synonym of 'travel')
'enter the capital of germany' is the wordplay.
'enter' becomes 'go in' (synonyms).
'the capital of' suggests taking the first letters.
The initial letter of 'germany' is 'g'.
'goin'+'g'='GOING'
'when' acts as a link.
(Other definitions for going that I've seen before include "fading" , "in action" , "Moving away, departing" , "State of turf" , "Running; leaving" .)