Possible Clues:

• Island in Caribbean, as opposed to one in Pacific? Not quite
• Its leading city is St. John's
• Island Columbus reached in 1493
• Important I guarantee to provide accommodation for holiday destination
• Caribbean island where iguana dances round top of tree
• Island named by Columbus in 1493
• Barbuda's partner island
• The name of this Caribbean island is Spanish for “ancient”
• Island worker on one other island briefly
• Island occupant I guarantee to protect