Sentimental feeling again lost at sea (9)
I believe the answer is:
nostalgia
'sentimental feeling' is the definition.
(I've seen this before)
'again lost at sea' is the wordplay.
'at sea' indicates anagramming the letters.
'again'+'lost'='againlost'
'againlost' with letters rearranged gives 'NOSTALGIA'.
(Other definitions for nostalgia that I've seen before include "Longing for what's gone" , "Sentimental longing for the past" , "fondly looking back" , "Remembrance of Things Past" , "dreaming of the past" .)