The thing is, one of the bits is missing! (3)
I believe the answer is:
its
'the thing is one' is the definition.
I know nothing about this answer so I cannot judge whether this works.
'of the bits is missing' is the wordplay.
'of' indicates a hidden word.
'ITS' is hidden within 'bits missing'.
Can you help me to learn more?
(Other definitions for its that I've seen before include "The thing's" , "Neuter possessive pronoun" , "Belonging to a particular thing" , "Another form of 'tis" , "Sit (anag.)" .)