In retrospect, realise it's a ghost (7)
I believe the answer is:
spectre
'ghost' is the definition.
(synonyms)
'in retrospect realise it's a' is the wordplay.
'in' indicates a hidden word.
'SPECTRE' is hidden within 'retrospect realise a'.
(Other definitions for spectre that I've seen before include "Ghostly figure" , "Enemy organisation for James Bond; ghost" , "Wraith" , "something seen as a threat" , "Respect for a ghost" .)