At last, tropic zone gets a royal ruler (4)
I believe the answer is:
czar
'ruler' is the definition.
(czar is a kind of ruler)
'at last tropic zone gets a royal' is the wordplay.
'at last' suggests the final letters.
'zone' becomes 'z'.
'gets' is a charade indicator (letters next to each other).
'royal' becomes 'r' (abbreviation).
The final letter of 'tropic' is 'c'.
'c'+'z'+'a'+'r'='CZAR'
(Other definitions for czar that I've seen before include "person of great power" , "Russian emperor" , "Pre-1917 Russian ruler" , "Despot" , "policy coordinator" .)