It’s made to sit up (3)
I believe the answer is:
tis
'it's made' is the definition.
I can't tell whether this definition defines the answer.
'sit up' is the wordplay.
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'sit' backwards is 'TIS'.
'to' acts as a link.
Can you help me to learn more?
(Other definitions for tis that I've seen before include "Contraction of it is" , "Sequel to "Angela's Ashes"" , "McCourt work" , "For poet, that's" , "That thing's" .)