One form of eternity (3)
I believe the answer is:
eon
'eternity' is the definition.
(I've seen this in another clue)
'one form' is the wordplay.
'form' is an anagram indicator.
'one' anagrammed gives 'EON'.
'of' acts as a link.
(Other definitions for eon that I've seen before include "Very long age" , "Major division of geological time" , "Huge time period" , "thousand million years" , "Prolonged period" .)