Left because of almost nothing (4)
I believe the answer is:
love
'nothing' is the definition.
(love means zero in tennis)
'left because of almost' is the wordplay.
'left' becomes 'l' (common abbreviation).
'because of' becomes 'over' (eg 'he's upset over it').
'almost' means to remove the last letter (most of the word but not all of it).
'over' with its last letter taken off is 'ove'.
'l'+'ove'='LOVE'
(Other definitions for love that I've seen before include "Zero score, dear" , "1970 film, starring Ali MacGraw and Ryan O'Neal" , "On court, zero" , "See 10" , "Score of zero at tennis" .)