Long story about end of Empire (5)
I believe the answer is:
yearn
'long' is the definition.
(I know that long can be written as yearn)
'story about end of empire' is the wordplay.
'story' becomes 'yarn' (synonyms).
'about' means one lot of letters goes inside another.
'end of' suggests the final letters.
The final letter of 'empire' is 'e'.
'yarn' enclosing 'e' is 'YEARN'.
(Other definitions for yearn that I've seen before include "Curdle" , "Ache, crave" , "wish" , "Hanker" , "Have strong desire" .)