Spook is good company (5)
I believe the answer is:
ghost
'spook' is the definition.
(I know that spook can be written as ghost)
'good company' is the wordplay.
'good' becomes 'g' (abbreviation).
'company' becomes 'host' (I can't explain this - if you can you should believe this answer much more).
'g'+'host'='GHOST'
'is' acts as a link.
Can you help me to learn more?
(Other definitions for ghost that I've seen before include "A spirit from the beyond" , "Hamlet's spectral father" , "Spectre, wraith" , "Author, secretly" , "author for someone" .)