Stuck on:
18d At twenty-one I succeeded in arrangement for a loan (6) ??R?ST
prest or imprest is a loan but can'r see how they would fit
21a Past ruler is disqualified in stages from eighteen (4) ?R?T
perhaps these 2 are connected ?
12d Prison van to stop, anticipating shut-eye (6) SL?M??
5th letter could be b or e ... giving slumber or slammer ???