One sort of period (3)
I believe the answer is:
eon
'period' is the definition.
(eon is a kind of period)
'one sort' is the wordplay.
'sort' is an anagram indicator.
'one' is an anagram of 'EON'.
'of' is the link.
(Other definitions for eon that I've seen before include "Very long age" , "Major division of geological time" , "Long period of time" , "Vast age; eternity" , "Era" .)