Ponder endlessly about problem (3)
I believe the answer is:
sum
'problem' is the definition.
(arithmetic problem)
'ponder endlessly about' is the wordplay.
'ponder' becomes 'muse' (synonyms).
'endlessly' means to remove the last letter.
'about' says the letters should be written backwards.
'muse' with its final letter taken away is 'mus'.
'mus' written backwards gives 'SUM'.
(Other definitions for sum that I've seen before include "Quantity of money" , "maths problem" , "Amount of money; total" , "Arithmetic calculation" , "Amount, of money say" .)