Some very early period of time (4)
I believe the answer is:
year
'time' is the definition.
(year is a unit of time)
'some very early period' is the wordplay.
'some' says the answer is hidden in the clue (some parts of the word are used).
'YEAR' is hidden within 'very early period'.
'of' acts as a link.
(Other definitions for year that I've seen before include "Four seasons" , "One orbit round the sun" , "Period of 5 across" , "Time period" , "Period of revolution" .)