'Tis merely another form (3)
I believe the answer is:
its
''tis' is the definition.
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for its that I've seen before include "Sit (anag.)" , "Belonging to something nonhuman" , "Another form of 'tis" , "Neuter possessive pronoun" , "The thing's" .)