One cross-channel article and another occupying leasing firm (11)

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

UN (=cross-channel article, i.e. a French word for a ) + [LE (=cross-channel article, i.e. a French word for the ) in RENTING (=leasing)]; unrelenting is firm, as an adjective, steadfast