Father had put on a serious face (7)
I believe the answer is:
frowned
'put on a serious face' is the definition.
'father had' is the wordplay.
'father' becomes 'fr'.
'had' becomes 'owned' ('own' can be a synonym of 'have').
'fr'+'owned'='FROWNED'
(Other definitions for frowned that I've seen before include "looked worried" , "Wrinkled one's forehead" , "Glowered" .)