Lost again, transported by 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.
'lost again transported' is the wordplay.
'transported' indicates anagramming the letters.
'lost'+'again'='lostagain'
'lostagain' with letters rearranged gives 'NOSTALGIA'.
'by' acts as a link.
(Other definitions for nostalgia that I've seen before include "Sentimental feeling for old times" , "Longing for something past - isn't a goal (anag)" , "Sentimental longing for the past" , "Fond remembrance" , "Sense of past perfect" .)