1521 visitor to Guam (8)
I believe the answer is:
magellan
'1521 visitor to guam' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for magellan that I've seen before include "Port. explorer; a S. Am. Strait" , "Strait between Atlantic and Pacific oceans" , "Major strait between the Atlantic and Pacific" , "Portuguese navigator who crossed the Pacific" , "Ferdinand -, Portuguese navigator" .)