Campaign ad covers front of office in maroon (7)

I haven't cracked this one yet — but the brilliant team at Fifteensquared have:

CRUS [o] E (maroon, as a noun = a person marooned on an island, such as Robinson Crusoe), with AD replacing (covering) the O which is the front letter of “office”. I hadn’t come across this noun sense of “maroon” before, but both Chambers and Collins recognise it, though the latter says the usage is “rare”.