Period of time, long, endless? (4)
I believe the answer is:
year
'period of time' is the definition.
(year is a kind of period of time)
'long endless?' is the wordplay.
'long' becomes 'yearn' ('yearn' can be a synonym of 'long').
'endless?' means to remove the last letter (I've seen 'endless' mean this).
'yearn' with its last letter taken away is 'YEAR'.
(Other definitions for year that I've seen before include "TV advert" , "Time taken by a planet to orbit the sun" , "Full calendar?" , "Four seasons" , "Jan to Dec" .)