The I in TGIF (3)
I believe the answer is:
its
'the i in tgif' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for its that I've seen before include "Sit (anag.)" , "Another form of 'tis" , "Belonging to something nonhuman" , "Belonging to a particular thing" , "Neuter possessive pronoun" .)