A long time finding Eeyore's age? (7,5)
I believe the answer is:
Here is my best explanation:
'a long time' is the definition.
'eeyore's age?' is the subsidiary indication.
'eeyore's' becomes 'donkeys' (eeyore is the donkey from winnie the pooh).
'age?' becomes 'years' ('years' can be a synonym of 'age').
'finding' is the link.
(Other definitions for donkeys years that I've seen before include "A beastly long time", "Ages and ages", "Yonks", "An age", "Beastly long time", "forever", "A long time (spent on Blackpool beach?)", "What's evident in old ass" and "Very long time".)