Not the part of the ghost but an important one (5,4)

I believe the answer is:

vital role

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'the ghost but an important one' is the definition.
I know nothing about this answer so I can't judge whether it can be defined by this definition.

'not the part' is the wordplay.
'not' becomes 'vital' (I am not sure about this - if you are sure you should believe this answer much more).
'the part' becomes 'role' (eg a theatrical role).
'vital'+'role'='VITAL ROLE'

'of' acts as a link.

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