Comes across holding up bag (4)
I believe the answer is:
case
'bag' is the definition.
(I have seen 'Overnight bag' mean 'case' so perhaps 'bag' could also mean 'case')
'comes across holding up' is the wordplay.
'holding' indicates the answer is hidden within the clue.
'up' is a reversal indicator (in down clue: letters go upwards).
'esac' is hidden within 'comes across'.
'esac' back-to-front is 'CASE'.
(Other definitions for case that I've seen before include "Legal action; container" , "Patient - luggage" , "Humorous character" , "Check out" , "Legal action; box of wine" .)