He mapped the source of the Orinoco after raising one of London's hospitals (6)

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

A charade of STRAB, a reversal (‘raising’ in a down light) of BARTS (St Bartholemew’s, ‘one of London’s hospitals’) plus O (‘the source of the O rinoco’). Strabo was an ancient Greek geographer who travelled widely, but not to the Orinoco.