Sentimental longing (9)
I believe the answer is:
nostalgia
'sentimental longing' is the definition.
'nostalgia' can be an answer for 'longing' (nostalgia is a kind of longing). I also know that 'sentimental' relates to this answer.
This is all the clue.
(Other definitions for nostalgia that I've seen before include "longing for home and family" , "Fond memories" , "Sense of past perfect" , "That may bring on homesickness" , "Longing for what's gone" .)