Trend of opinion indicating change of diet (4)
I believe the answer is:
tide
'trend of opinion' is the definition.
'change of diet' is the wordplay.
'change of' is an anagram indicator.
'diet' is an anagram of 'TIDE'.
'indicating' is the link.
(Other definitions for tide that I've seen before include "Season; current" , "It ebbs and flows" , "Time and it wait for no man" , "What might carry" , "In the main, movement" .)