Sentimental feeling unfortunately lost again (9)
I believe the answer is:
nostalgia
'sentimental feeling' is the definition.
(I've seen this before)
'unfortunately lost again' is the wordplay.
'unfortunately' is an anagram indicator (a bad or unfortunate spelling).
'lost'+'again'='lostagain'
'lostagain' with letters rearranged gives 'NOSTALGIA'.
(Other definitions for nostalgia that I've seen before include "Sense of past perfect" , "That may bring on homesickness" , "Longing for something past - isn't a goal (anag)" , "Fond remembrance" , "Love for the past" .)