Essential part of opening play is the hype (4)
I believe the answer is:
pith
'essential part' is the definition.
(I've seen this before)
'opening play is the hype' is the wordplay.
'opening' says to take the initial letters.
The first letters of 'play is the hype' is 'PITH'.
'of' acts as a link.
(Other definitions for pith that I've seen before include "Central part" , "Marrow (of plant)" , "nub of matter" , "Inner skin of citrus fruit" , "Tissue inside the rind of citrus fruits" .)