Tis indeed part of Whitsuntide (3)
I believe the answer is:
its
'tis indeed part' is the definition.
I know nothing about this answer so I cannot judge whether it can be defined by this definition.
'of whitsuntide' is the wordplay.
'of' says the answer is hidden in the clue.
'ITS' can be found hidden inside 'whitsuntide'.
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 a particular thing" , "Neuter possessive pronoun" , "Belonging to something nonhuman" .)