The extent to which diets can vary (8)
I believe the answer is:
distance
'the extent' is the definition.
(I have seen 'The extent' mean 'distance' so perhaps 'extent' could also mean 'distance')
'diets can vary' is the wordplay.
'vary' indicates anagramming the letters.
'diets'+'can'='dietscan'
'dietscan' with letters rearranged gives 'DISTANCE'.
'to which' is the link.
(Other definitions for distance that I've seen before include "The gap between two places" , "Space between two objects or points" , "Space between things" , "Aloofness, reserve" , "league, for example" .)