Nothing left in back (3)
I believe the answer is:
nil
'nothing' is the definition.
(eg nil is zero in football)
'left in back' is the wordplay.
'left' becomes 'l' (common abbreviation).
'back' shows that the letters should be reversed in order.
'l'+'in'='lin'
'lin' back-to-front is 'NIL'.
(Other definitions for nil that I've seen before include "Nothing, naught" , "Latin zero" , "Score of nought" , "Nothing, zilch" , "Nothing found in senility" .)