Unknown barge travels into capital (6)
I believe the answer is:
zagreb
'capital' is the definition.
(capital of Croatia)
'unknown barge travels' is the wordplay.
'unknown' becomes 'z' (algebra).
'travels' is an anagram indicator.
'barge' is an anagram of 'agreb'.
'z'+'agreb'='ZAGREB'
'into' is the link.
(Other definitions for zagreb that I've seen before include "Croatia's capital city" , "Croatian city" , "European capital" , "on River Sava" , "European city" .)