'Tis wrong to sit badly (3)
I believe the answer is:
its
''tis' is the definition.
'wrong to sit badly' is the wordplay.
I cannot quite see how this works, but
an anagram of 'sit' is 'its' which is within the answer.
No letters remain.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for its that I've seen before include "Another form of 'tis" , "The thing's" , "Belonging to something nonhuman" , "Sit (anag.)" , "Neuter possessive pronoun" .)