Foolishly lost again in yearning for the past (9)
I believe the answer is:
nostalgia
'yearning for the past' is the definition.
(I've seen this before)
'foolishly lost again' is the wordplay.
'foolishly' indicates anagramming the letters.
'lost'+'again'='lostagain'
'lostagain' anagrammed gives 'NOSTALGIA'.
'in' acts as a link.
(Other definitions for nostalgia that I've seen before include "Sentimental longing for the past" , "A sentimental longing of analogist" , "That may bring on homesickness" , "dreaming of the past" , "Attraction to the past" .)