There's nothing in turning fifty (3)
I believe the answer is:
nil
'there's nothing' is the definition.
(nil means a score of zero in football)
'in turning fifty' is the wordplay.
'turning' shows that the letters should be reversed in order.
'fifty' becomes 'l' (Roman numeral).
'in' written backwards gives 'ni'.
'ni'+'l'='NIL'
(Other definitions for nil that I've seen before include "Don't despair - . . . desperandum" , "0" , "You get nothing" , "There's nothing left" , "Nothing, no value" .)