My retired era, beginning in Disneyland (4)
I believe the answer is:
egad
'my' is the definition.
(I've seen this before)
'retired era beginning in disneyland' is the wordplay.
'retired' shows that the letters should be reversed in order.
'era' becomes 'age' (both can mean a long period of time).
'beginning in' indicates taking the first letters.
The first letter of 'disneyland' is 'd'.
'age' backwards is 'ega'.
'ega'+'d'='EGAD'
(Other definitions for egad that I've seen before include "Archaic expression of anger, surprise, etc" , "Forgotten curse" , "Oath sworn" , "Christ" , "Old oath" .)