He mapped the source of the Orinoco after raising one of London's hospitals (6)
I believe the answer is:
strabo
'he mapped' is the definition.
I don't know anything about this answer so I can't judge whether this works.
'source of the orinoco after raising one of london's hospitals' is the wordplay.
'source of' suggests taking the first letters (the start - or source - of the word).
'after' means one lot of letters go next to another.
'raising' is a reversal indicator (in a down clue, letters go up).
'one of london's hospitals' becomes 'barts' (St Bartholomew's Hospital).
The first letter of 'orinoco' is 'o'.
'barts' backwards is 'strab'.
'o' after 'strab' is 'STRABO'.
'the' acts as a link.
Can you help me to learn more?