Point back to 500 (3)
I believe the answer is:
dot
'point' is the definition.
(I know that point can be written as dot)
'back to 500' is the wordplay.
'back' shows that the letters should be reversed in order.
'500' becomes 'D' (Roman numeral for 500).
'to'+'d'='tod'
'tod' reversed gives 'DOT'.
(Other definitions for dot that I've seen before include "'Spot, tiny mark (3)'" , "Small round spot" , "Speck; dowry" , "One of the signals in Morse code" , "One signal used in Morse code" .)