Learned to rue diet changes (7)
I believe the answer is:
erudite
'learned' is the definition.
(erudite means learned or well-read)
'rue diet changes' is the wordplay.
'changes' indicates an anagram.
'rue'+'diet'='ruediet'
'ruediet' with letters rearranged gives 'ERUDITE'.
'to' is the link.
(Other definitions for erudite that I've seen before include "Learned, well educated" , "Showing profound knowledge" , "Full of knowledge" , "Well educated, having great learning" , "'Learned, knowledgeable (7)'" .)