Chew the cud as friend consumes mixed diet (8)
I believe the answer is:
meditate
'chew the cud' is the definition.
(thesaurus)
'friend consumes mixed diet' is the wordplay.
'friend' becomes 'mate' (mate is a kind of friend).
'consumes' means one lot of letters goes inside another.
'mixed' indicates anagramming the letters.
'diet' is an anagram of 'edit'.
'mate' enclosing 'edit' is 'MEDITATE'.
'as' acts as a link.
(Other definitions for meditate that I've seen before include "Ruminate" , "Muse" , "Reflect upon" , "Think intently and at length" , "Cogitate" .)