Narrow shoal outside large Croatian port (5)
I believe the answer is:
split
'croatian port' is the definition.
(I've seen this before)
'narrow shoal outside large' is the wordplay.
'narrow shoal' becomes 'spit' (I've seen this in another clue).
'outside' means one lot of letters goes inside another (some letters go outside others).
'large' becomes 'L' (eg in clothes sizes).
'spit' enclosing 'l' is 'SPLIT'.
(Other definitions for split that I've seen before include "Break, fracture" , "Break in pieces - leave" , "Divide, break apart" , "Divide - Croatian port" , "Diverge" .)