Heedless drunk scratching bottom in open (9)
I believe the answer is:
oblivious
'heedless' is the definition.
(I've seen this before)
'drunk scratching bottom in open' is the wordplay.
'drunk' becomes 'lit' (I've seen this in other clues).
'scratching bottom' means to remove the last letter.
'in' indicates putting letters inside.
'open' becomes 'obvious' (similar in meaning).
'lit' with its final letter taken away is 'li'.
'li' placed within 'obvious' is 'OBLIVIOUS'.
(Other definitions for oblivious that I've seen before include "Apt to forget" , "Totally unaware" , "Blind (to)" , "Heedless" , "totally in the dark?" .)