Sit up when it is abbreviated (3)
I believe the answer is:
tis
'when it is abbreviated' is the definition.
I can't judge whether this definition defines the answer.
'sit up' is the wordplay.
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'sit' backwards is 'TIS'.
Can you help me to learn more?
(Other definitions for tis that I've seen before include "Contraction of it is" , "For poet, that's" , "Sequel to "Angela's Ashes"" , "... the season to be jolly" , "It is shortened" .)