Long short time (4)
I believe the answer is:
year
'time' is the definition.
(I know that time can be written as year)
'long short' is the wordplay.
'long' becomes 'yearn' (synonyms).
'short' means to remove the last letter.
'yearn' with its last letter removed is 'YEAR'.
(Other definitions for year that I've seen before include "Student group" , "365 or 366 days" , "Time taken by a planet to orbit the sun" , "1961?" , "Time period" .)