Current variation in diet (4)
I believe the answer is:
tide
'current' is the definition.
(I've seen this in another clue)
'variation in diet' is the wordplay.
'variation in' is an anagram indicator.
'diet' with letters rearranged gives 'TIDE'.
(Other definitions for tide that I've seen before include "turning point" , "Season; current" , "In the main, movement" , "Time and it wait for no man" , "Ebb and flow of water" .)