It's from e.g. Tiber estuary, not Rhode Island (7,8)
I believe the answer is:
italian vermouth
'it's' is the definition.
(I've seen this before)
'e g tiber estuary not rhode island' is the wordplay.
I cannot quite see how this works, but
'e' is present in the answer.
'g' could be 'm' (synonyms) and 'm' is found in the answer.
'not' could be 'o' (I've seen this in other clues) and 'o' is found within the answer.
'island' could be 'i' (geographical abbreviation) and 'i' is located in the answer.
This explanation may well be incorrect...
'from' is the link.
Can you help me to learn more?
(Other definitions for italian vermouth that I've seen before include "Drink" , "It expanded" , "two words for it!" .)