One with dilapidated mansion revealing inability to sleep (8)
I believe the answer is:
insomnia
'inability to sleep' is the definition.
(I've seen this before)
'one with dilapidated mansion' is the wordplay.
'one' becomes 'i' (Roman numeral).
'with' means one lot of letters go next to another.
'dilapidated' indicates anagramming the letters.
'mansion' is an anagram of 'nsomnia'.
'i'+'nsomnia'='INSOMNIA'
'revealing' is the link.
(Other definitions for insomnia that I've seen before include "Habitual inability to sleep (8)" , "Condition affecting sleep" , "In restless state" , "Sleep disorder" , "Wakefulness" .)