Passing through capital of Turkey change trains (7)
I believe the answer is:
transit
'passing' is the definition.
(transit can mean passing or travelling across)
'capital of turkey change trains' is the wordplay.
'capital of' suggests taking the first letters.
'change' indicates an anagram.
The first letter of 'turkey' is 't'.
'trains' is an anagram of 'ransit'.
't'+'ransit'='TRANSIT'
'through' is the link.
(Other definitions for transit that I've seen before include "carriage" , "Act of passing across" , "Travel" , "Pass across" , "To pass through/across" .)