Longing to sing, a lass not half excited (9)
I believe the answer is:
nostalgia
'longing' is the definition.
(nostalgia is a kind of longing)
'to sing a lass not half excited' is the wordplay.
'not half' indicates taking half.
'excited' indicates an anagram.
'lass' cut in half is 'la'.
'to'+'sing'+'a'+'la'='tosingala'
'tosingala' is an anagram of 'NOSTALGIA'.
(Other definitions for nostalgia that I've seen before include "longing for home and family" , "Sentimental feeling of analogist" , "Attraction to the past" , "Yearning for past - homesickness" , "A sentimental longing of analogist" .)