Understanding myth pays off (8)
I believe the answer is:
sympathy
'understanding' is the definition.
(I know that understanding can be written as sympathy)
'myth pays off' is the wordplay.
'off' is an anagram indicator.
'myth'+'pays'='mythpays'
'mythpays' is an anagram of 'SYMPATHY'.
(Other definitions for sympathy that I've seen before include "Fellow-feeling in time of trouble" , "agreement" , "Pity; common feeling" , "Feeling like another" , "comfort" .)