Long story describing the end of empire (5)
I believe the answer is:
yearn
'long' is the definition.
('yearn' can be a synonym of 'long')
'story describing the end of empire' is the wordplay.
'story' becomes 'yarn' ('yarn' can be a synonym of 'story').
'describing' is an insertion indicator (describe can mean to form the outline of).
'the end of' indicates one should take 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 "Pant - pine" , "Ache, crave" , "Hanker" , "lust" , "Mark of injury" .)