It's different when you sit up (3)
I believe the answer is:
tis
'you sit up' is the definition.
I can't tell whether this definition defines the answer.
'it's different' is the wordplay.
'different' indicates anagramming the letters (letters placed in a different order).
'its' anagrammed gives 'TIS'.
'when' 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" , "That thing's" , "It is shortened" , "McCourt work" , "For poet, that's" .)