Dig melon out -- get rich from this! (4,4)
I believe the answer is:
gold mine
'get rich from this' is the definition.
(a gold mine is lucrative opportunity)
'dig melon out' is the wordplay.
'out' is an anagram indicator (out can mean wrong or inaccurate).
'dig'+'melon'='digmelon'
'digmelon' anagrammed gives 'GOLD-MINE'.
(Other definitions for gold mine that I've seen before include "Source of great wealth" , "Source of great riches" .)