Japanese wrestling is endless problem (3)
I believe the answer is:
sum
'problem' is the definition.
(arithmetic problem)
'japanese wrestling is endless' is the wordplay.
'japanese wrestling' becomes 'sumo' (I've seen this before).
'is endless' means to remove the last letter.
'sumo' with its final letter taken off is 'SUM'.
(Other definitions for sum that I've seen before include "Mathematical problem" , "An additional problem" , "All one has" , "Amount of money; total" , "Amount, aggregate" .)