Very old structure from the back spooks (7)
I believe the answer is:
archaic
'very old' is the definition.
(I've seen this before)
'structure from the back spooks' is the wordplay.
I cannot quite see how this works, but
'structure' could be 'arch' (arch is a kind of structure) and 'arch' is found in the answer.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for archaic that I've seen before include "Belonging to former times, perhaps word usage" , "Belonging to former or ancient times" , "Ancient, no longer current" , "Very old or old-fashioned" , "Antiquated" .)