No good megarich, sad monster (7)
I believe the answer is:
chimera
'monster' is the definition.
(Chimera is an example)
'no good megarich sad' is the wordplay.
'no' is a deletion indicator.
'good' becomes 'g' (abbreviation).
'sad' indicates an anagram.
'megarich' with 'g' taken out is 'mearich'.
'mearich' with letters rearranged gives 'CHIMERA'.
(Other definitions for chimera that I've seen before include "wild dream" , "Fantastic creation of the mind, like Greek mythical monster" , "fabulous creature" , "Wild or impossible idea" , "phantom" .)