Anchor held by hideous, headless phantom (7)
I believe the answer is:
ghostly
'phantom' is the definition.
(phantom can mean ghostly or unreal)
'anchor held by hideous headless' is the wordplay.
'anchor' becomes 'host ' (I've seen this before).
'held by' indicates putting letters inside.
'hideous' becomes 'ugly' (similar in meaning).
'headless' means to remove the first letter.
'ugly' with its initial letter removed is 'gly'.
'host' put within 'gly' is 'GHOSTLY'.
(Other definitions for ghostly that I've seen before include "Phantom-like, spectral" , "Phantasmal" , "Eerie, unnatural" , "Spectral, like Marley" .)