The age of galleons (3)
I believe the answer is:
eon
'the age' is the definition.
(eon is a kind of age)
'of galleons' is the wordplay.
'of' indicates the answer is hidden within the clue.
'EON' is hidden in the letters of 'galleons'.
(Other definitions for eon that I've seen before include "Long period of time" , "Vast period" , "Vast age; eternity" , "Very long period (US and technical spelling)" , "Geological period" .)