Old horse of Golden Age (3)
I believe the answer is:
nag
'old horse' is the definition.
(I've seen this before)
'of golden age' is the wordplay.
'of' indicates a hidden word.
'NAG' is hidden in the letters of 'golden age'.
(Other definitions for nag that I've seen before include "Complain, worry" , "Poor horse" , "Henpeck like old horse" , "Constantly scold - small horse" , "Old horse or shrewish wife" .)