Not smiling back (5)
I believe the answer is:
stern
This could be a double definition.
'not smiling' is the first definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
'back' is the second definition.
(stern means the back of a boat)
Can you help me to learn more?
(Other definitions for stern that I've seen before include "Authoritarian, back" , "having a forbidding appearance" , "Rigorous - back of boat" , "Forbidding, austere" , "Back end of a ship" .)