The length of time around it varies (8)
I believe the answer is:
duration
'the length of time' is the definition.
(I've seen this before)
'around it varies' is the wordplay.
'varies' indicates anagramming the letters.
'around'+'it'='aroundit'
'aroundit' with letters rearranged gives 'DURATION'.
(Other definitions for duration that I've seen before include "Time scale" , "Length of time" , "Period during which something continues" , "Term, span, time" , "Around it (anag.)" .)