Scroungers and rotters holding what's on plate up (7)
I believe the answer is:
cadgers
'scroungers' is the definition.
('cadger' can be a synonym of 'scrounger')
'rotters holding what's on plate up' is the wordplay.
'rotters' becomes 'cads' (I've seen this before).
'holding' is an insertion indicator.
'what's on plate' becomes 'reg' (a number plate shows a car's registration number).
'up' is a reversal indicator (in down clue: letters go upwards).
'reg' in reverse letter order is 'ger'.
'cads' placed around 'ger' is 'CADGERS'.
'and' is the link.
(Another definition for cadgers that I've seen is " Beggars".)