Sum 500 + 500 + 1? (3,2)
I believe the answer is:
add up
'sum' is the definition.
(synonyms)
'500 500 1?' is the wordplay.
I cannot really understand how this works, but
'500' could be 'd' (Roman numeral for 500) and 'd' is found within the answer.
'1?' could be 'a' (both can mean one) and 'a' is found in the answer.
The remaining letters 'dup' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for add up that I've seen before include "Seem to be sensible" , "Make sense; count" , "Sum - make sense" , "Put together (numbers)" , "Reckon" .)