Endlessly long time (4)
I believe the answer is:
year
'time' is the definition.
(year is a unit of time)
'endlessly long' is the wordplay.
'endlessly' means to remove the last letter.
'long' becomes 'yearn' (synonyms).
'yearn' with its last letter removed is 'YEAR'.
(Other definitions for year that I've seen before include "1381 say" , "Full calendar?" , "See 33 Across" , "Orbital period" , "Period of twelve months" .)