Hard to believe that is 500 squared (7)
I believe the answer is:
tallied
'squared' is the definition.
The definition and answer can be both related to competing as well as being past participle verbs.
Maybe there's an association between them I don't understand?
'hard to believe that is 500' is the wordplay.
'hard to believe' becomes 'tall' (as in a tall tale).
'that is' becomes 'ie' (abbreviation for id est, 'that is' in Latin).
'500' becomes 'D' (Roman numeral for 500).
'tall'+'ie'+'d'='TALLIED'
Can you help me to learn more?
(Other definitions for tallied that I've seen before include "Reckoned debt or score" , "Corresponded - kept score" , "Agreed, corresponded" , "Agreed; calculated" , "Matched" .)