Where Eagles Dare? Not All Their Yesterdays, on reflection (5)
I believe the answer is:
eyrie
'where eagles dare?' is the definition.
The answer is a location as well as being a singular noun. This is suggested by the definition.
'not all their yesterdays on reflection' is the wordplay.
'not all' indicates the answer is hidden within the clue (only some of the letters).
'on reflection' shows that the letters should be reversed in order.
'eirye' is hidden in the letters of 'their yesterdays'.
'eirye' in reverse letter order is 'EYRIE'.
Can you help me to learn more?
(Other definitions for eyrie that I've seen before include "home hard to reach" , "lofty house" , "high point" , "Inaccessible place" , "Bird of prey's high nest" .)