Longest of a group of islands off the coast of Florida (3,5)
I believe the answer is:
key largo
'islands off the coast of florida' is the definition.
I know nothing about this answer so I cannot judge whether this works.
'longest of a group of' is the wordplay.
I cannot really see how this works, but
'of' could be 'o' and 'o' is found in the answer.
'a' is found in the answer.
'group of' could be 'g' (eg the G7 nations, Group of Seven) and 'g' is found within the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for key largo that I've seen before include "film" , "parts here for 1a and 9 [BACALL & BOGART] - with 12a" , "island" .)