Long period ending in autumn (5)
I believe the answer is:
yearn
'long' is the definition.
(I know that long can be written as yearn)
'period ending in autumn' is the wordplay.
'period' becomes 'year' (year is a kind of period).
'ending in' indicates one should take the final letters.
The last letter of 'autumn' is 'n'.
'year'+'n'='YEARN'
(Other definitions for yearn that I've seen before include "Ache, crave" , "Have an intense feeling of longing" , "Have a craving" , "Long (for something)" , "Desire, want" .)