Ghastly Blunt, essentially overcome by terrible flaw (5)
I believe the answer is:
awful
'ghastly' is the definition.
(I've seen this in another clue)
'blunt essentially overcome by terrible flaw' is the wordplay.
'essentially' says to take the centre.
'overcome by' means one lot of letters goes inside another.
'terrible' indicates an anagram.
The middle letter of 'blunt' is 'u'.
'flaw' is an anagram of 'awfl'.
'u' put inside 'awfl' is 'AWFUL'.
(Other definitions for awful that I've seen before include "pants" , "Very poor" , "Horrific" , "Fearsome" , "shocking" .)