Letter to a sailor in a Gulf state (5)
I believe the answer is:
qatar
'gulf state' is the definition.
(I've seen this in another clue)
'letter to a sailor' is the wordplay.
'letter' becomes 'q' ().
'to' means one lot of letters go next to another.
'sailor' becomes 'tar' (tar can mean a sailor).
'q'+'a'+'tar'='QATAR'
'in a' acts as a link.
(Other definitions for qatar that I've seen before include "Gulf sheikhdom" , "Arab peninsula" , "Doha's state" , "Sporting venue" , "Hot state" .)