Existence of some fairies -- seriously! (4)
I believe the answer is:
esse
'existence' is the definition.
(I've seen this before)
'some fairies seriously' is the wordplay.
'some' says the answer is hidden in the clue (some of the parts of the word are used).
'ESSE' is hidden in the letters of 'fairies seriously'.
'of' is the link.
(Other definitions for esse that I've seen before include "Fundamental nature (philosophy)" , "Actual existence" , "Reality" , "Sees (anag.)" , "Essential nature (philosophy)" .)