Dark ultimately — terribly early start — to get to US island (3,5)
I believe the answer is:
key largo
'to us island' is the definition.
(island in Florida)
'dark ultimately terribly early start' is the wordplay.
'ultimately' says to take the final letters.
'terribly' indicates an anagram.
'start' becomes 'go' (synonyms).
The final letter of 'dark' is 'k'.
'early' anagrammed gives 'eylar'.
'k'+'eylar'+'go'='KEY LARGO'
'to get' is the link.
(Other definitions for key largo that I've seen before include "island" , "film" , "parts here for 1a and 9 [BACALL & BOGART] - with 12a" .)