Vet drops in on spook (7)
I believe the answer is:
spectre
'spook' is the definition.
(both can mean a ghost)
'vet drops in on' is the wordplay.
'vet' becomes 'inspect' (inspect can mean to vet or check).
'drops' is a deletion indicator.
'on' becomes 're' ('on' can be mean 'with reference to').
'inspect' with 'in' taken out is 'spect'.
'spect'+'re'='SPECTRE'
(Other definitions for spectre that I've seen before include "Wraith" , "Respect for a ghost" , "Ghost, phantom, apparition" , "Ghostly figure" , "Sceptre (anag.)" .)