Comes a cropper holding up killers (5)
I believe the answer is:
orcas
'killers' is the definition.
('orca' can be a synonym of 'killer')
'comes a cropper 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).
'sacro' is hidden in the letters of 'comes a cropper'.
'sacro' reversed gives 'ORCAS'.
(Other definitions for orcas that I've seen before include "Killer whales" , "Killers" .)