Admit being kept under by father displaying severe expression (5)
I believe the answer is:
frown
'severe expression' is the definition.
(severe facial expression)
'admit being kept under by father' is the wordplay.
'admit' becomes 'own' (own can mean to admit or acknowledge).
'being kept under by' means one lot of letters go next to another (in a down clue, some letters go below others).
'father' becomes 'fr'.
'own' put after 'fr' is 'FROWN'.
'displaying' is the link.
(Other definitions for frown that I've seen before include "make headlines?" , "Disapproving look" , "Glower" , "Facial expression of dislike or concentration" , "Scowl, look crossly" .)