Lost again in reverie about the past (9)
I believe the answer is:
nostalgia
'reverie about the past' is the definition.
(nostalgia is thinking about the past)
'lost again' is the wordplay.
'lost again' anagrams to 'NOSTALGIA'.
Though, I am unsure how the anagram is indicated.
'in' acts as a link.
(Other definitions for nostalgia that I've seen before include "Longing for something past - isn't a goal (anag)" , "Wistfulness" , "Sentimental longing for the past" , "Attraction to the past" , "severe homesickness" .)