Part of Croatian city on high (5,2)
I believe the answer is:
split up
'part' is the definition.
(I know that part can be written as split up)
'croatian city on high' is the wordplay.
'croatian city' becomes 'split' (I have seen 'European city' mean 'split' so perhaps 'city' could also mean 'split'. I am not sure about the 'croatian' bit.).
'on' means one lot of letters go next to another (in a down clue, letters appear on others).
'high' becomes 'up' (associated in meaning).
'split'+'up'='SPLIT UP'
'of' is the link.
Can you help me to learn more?
(Other definitions for split up that I've seen before include "Separate" , "Part company" , "Take apart" , "End a relationship" , "Divorce" .)