Guardian Bank Holiday Prize By Maskerade - Can't Parse 15A

[HughSW]

All done (and great fun), but can't parse 15a: Soldiers secure old city (7) I have O?L?N?O - I assume it is Orlando, but why?

[aelmpvw]

or land o
or = soldiers
land = secure
o = old

[Flonska]

OR (Ordinary Ranks) Land (to secure - land a fish) O (old)

[HughSW]

Thanks both!