I've got CELLIST. Prisons have cells and someone who is in a cell could be a cell-ist. That was my reasoning. Are you sure your 5d is correct? I've put ALEC, as in "smart Alec".
I think it must be cellist, so I must have the first letter wrong !
The clue was Smart man (4) which I thought was ABEL. Looks as though it has to be A?EC for cellist.
Which I guess would be ALEC
Thanks.