“Rent will be payable monthly in arrear”. This means the month’s rent is paid at the of the month end.
Contrast with “arrears”, which is money that is overdue, but as yet unpaid. Technically, you’re in default. So best not to confuse them, not that this will stop anyone.
A bit of a pedant’s trick for young players, I grant you — but mildly diverting on a slow afternoon.