There's nothing left in retrospect (3)
I believe the answer is:
nil
'there's nothing' is the definition.
(nil means a score of zero in football)
'left in retrospect' is the wordplay.
'left' says the letters should be written backwards.
'retrospect' becomes 'l' (I can't justify this - if you can you should believe this answer much more).
'in' written backwards gives 'ni'.
'ni'+'l'='NIL'
Can you help me to learn more?
(Other definitions for nil that I've seen before include "none" , "Nothing, no value" , "Score of nought" , "Zero, no score" , "Don't despair - . . . desperandum" .)