She's endlessly sad, being left behind (3)
I believe the answer is:
sal
'she's' is the definition.
'endlessly sad being left behind' is the wordplay.
'endlessly' means to remove the last letter.
'being left' becomes 'L' (common abbreviation).
'behind' says to put letters next to each other.
'sad' with its last letter taken off is 'sa'.
'sa'+'l'='SAL'
(Other definitions for sal that I've seen before include "Sarah (dim.)" , "- - volatile" , "girl?" .)