The face is both stern and very good (7)
I believe the answer is:
grimace
'the face' is the definition.
('grimace' can be a synonym of 'face')
'both stern and very good' is the wordplay.
'both stern' becomes 'grim' ('grim' can be a synonym of 'stern'. I am not sure about the 'both' bit.**).
'and' says to put letters next to each other.
'very good' becomes 'ace' (I've seen this before).
'grim'+'ace'='GRIMACE'
'is' is the link.
Can you help me to learn more?
(Other definitions for grimace that I've seen before include "Frown" , "Contort the face" , "Express wry amusement" , "(Make) a wry face" , "unpleasant expression" .)