Consider varying diet adopted by friend (8)
I believe the answer is:
meditate
'consider' is the definition.
(I've seen this before)
'varying diet adopted by friend' is the wordplay.
'varying' indicates an anagram.
'adopted by' is an insertion indicator (I've seen this in other clues).
'friend' becomes 'mate' (mate is a kind of friend).
'diet' with letters rearranged gives 'edit'.
'edit' going into 'mate' is 'MEDITATE'.
(Other definitions for meditate that I've seen before include "Think deeply and quietly" , "Relax mentally" , "brood" , "Muse" , "Think to relax one's mind" .)