Fail to notice some hanger-on giving up (6)
I believe the answer is:
ignore
'fail to notice' is the definition.
(synonyms)
'some hanger-on giving up' is the wordplay.
'some' indicates the answer is hidden within the clue (some of the parts of the word are used).
'up' says the letters should be written backwards (in down clue: letters go upwards).
'erongi' is hidden in the letters of 'hanger on giving'.
'erongi' backwards is 'IGNORE'.
(Other definitions for ignore that I've seen before include "Give no attention to" , "Pay no attention to, disdain" , "Pay no heed to" , "Fail to consider" , "Discount" .)