Age of galleons (3)
I believe the answer is:
eon
'age' is the definition.
(eon is a kind of age)
'of galleons' is the wordplay.
'of' says the answer is hidden in the clue.
'EON' is hidden within 'galleons'.
(Other definitions for eon that I've seen before include "Geological period" , "plenty of time" , "billion years" , "Prolonged period" , "Major division of geological time" .)