Down when up on day (3)
I believe the answer is:
sad
'down' is the definition.
(thesaurus)
'when up on day' is the wordplay.
'when' becomes 'as' (**).
'up' says the letters should be written backwards (in down clue: letters go upwards).
'on' means one lot of letters go next to another.
'day' becomes 'd'.
'as' backwards is 'sa'.
'sa'+'d'='SAD'
(Other definitions for sad that I've seen before include "grey" , "upset" , "Blue" , "Miserable" , "Melancholy, triste" .)