Endlessly long period of time (4)
I believe the answer is:
year
'time' is the definition.
(I've seen this before)
'endlessly long period' is the wordplay.
'endlessly' means to remove the last letter.
'long period' becomes 'years' (I've seen this before).
'years' with its last letter taken away is 'YEAR'.
'of' acts as a link.
(Other definitions for year that I've seen before include "Period of twelve months" , "365 or 366 days" , "Full calendar?" , "Period of just over 52 weeks" , "Jan to Dec" .)