Split, Croatia's top port? (5)
I believe the answer is:
cleft
'split' is the definition.
(I've seen this before)
'croatia's top port?' is the wordplay.
'top' says to take the initial letters.
'port?' becomes 'left' ('left' can be a synonym of 'port').
The first letter of 'croatia' is 'c'.
'c'+'left'='CLEFT'
(Other definitions for cleft that I've seen before include "Fissure; divided" , "Narrow gap" , "Split (as a stick)" , "Split, divided" , "Space" .)