Attitude adopted by oldest ancestors (6)
I believe the answer is:
stance
'attitude' is the definition.
(I know that stance is a type of attitude)
'adopted by oldest ancestors' is the wordplay.
'adopted by' says the answer is hidden in the clue.
'STANCE' can be found hidden inside 'oldest ancestors'.
(Other definitions for stance that I've seen before include "One's physical or mental attitude" , "Attitude or way of standing" , "position when batting" , "Position, at the tee perhaps" , "Posture; point of view" .)