Lots again unhappy, longing for the past (9)
I believe the answer is:
nostalgia
'longing for the past' is the definition.
(I've seen this before)
'lots again unhappy' is the wordplay.
'unhappy' is an anagram indicator.
'lots'+'again'='lotsagain'
'lotsagain' is an anagram of 'NOSTALGIA'.
(Other definitions for nostalgia that I've seen before include "Sentimental feeling of analogist" , "Longing for what's gone" , "Backward-looking sentiment" , "That may bring on homesickness" , "severe homesickness" .)