Most important part of Spithead (4)
I believe the answer is:
pith
'most important part' is the definition.
(I've seen this before)
'of spithead' is the wordplay.
'of' indicates the answer is hidden within the clue.
'PITH' can be found hidden inside 'spithead'.
(Other definitions for pith that I've seen before include "nub of matter" , "Inside skin of, say, orange" , "Lining of rind; essence" , "Orange product" , "Inner skin of citrus fruit" .)