1 + 500 + 500? Er … I can do this! (5)
I believe the answer is:
adder
'i can do this' is the definition.
The definition suggests a singular noun which matches the answer.
'1 500 500? er' is the wordplay.
'1' becomes 'a' (both can mean one).
'500' becomes 'd' (synonyms).
'500?' becomes 'd' (synonyms).
'a'+'d'+'d'+'er'='ADDER'
(Other definitions for adder that I've seen before include "An arithmetic snake" , "One contributing more" , "Something you have to reckon with" , "One creature" , "Small poisonous snake, viper" .)