Left because of almost nothing (4)
I believe the answer is:
love
'nothing' is the definition.
(love is 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 (almost the whole word is kept).
'over' with its final letter taken away 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" .)