Forlorn sweetheart almost alone in rendezvous (8)
I believe the answer is:
desolate
'forlorn' is the definition.
(synonyms)
'sweetheart almost alone in rendezvous' is the wordplay.
'sweetheart' becomes 'e' (the heart/centre of 'sweet').
'almost' means to remove the last letter (most of the word but not all of it).
'alone' becomes 'solo' (as in a musical solo).
'in' indicates putting letters inside.
'rendezvous' becomes 'date' (I've seen this before).
'solo' with its last letter taken away is 'sol'.
'e'+'sol'='esol'
'esol' placed inside 'date' is 'DESOLATE'.
(Other definitions for desolate that I've seen before include "Dispirited and lonely, being so elated" , "Lay waste, despoil" , "extremely sad" , "'Bleak, bare (8)'" , "Dale's toe (anag.)" .)