Ghastly Blunt, essentially overcome by terrible flaw (5)
I believe the answer is:
awful
'ghastly' is the definition.
(I've seen this before)
'blunt essentially overcome by terrible flaw' is the wordplay.
'essentially' means to look at the middle letters.
'overcome by' indicates putting letters inside.
'terrible' is an anagram indicator.
The middle letter of 'blunt' is 'u'.
'flaw' anagrammed gives 'awfl'.
'u' put into 'awfl' is 'AWFUL'.
(Other definitions for awful that I've seen before include "pants" , "Very poor" , "Horrific" , "Fearsome" , "shocking" .)