Croatia's foremost port is Split (5)
I believe the answer is:
cleft
'is split' is the definition.
('cleft' can be a synonym of 'split')
'croatia's foremost port' is the wordplay.
'foremost' suggests taking the first letters.
'port' becomes 'left' ('left' can be a synonym of 'port').
The initial letter of 'croatia' is 'c'.
'c'+'left'='CLEFT'
(Other definitions for cleft that I've seen before include "Split; fissure" , "Opening, fissure" , "Crack" , "chasm" , "Narrow opening" .)