South American in London area leaving 'as a precaution' (4)
I believe the answer is:
inca
'south american' is the definition.
(the Inca were a South American civilization)
'london area leaving as a precaution' is the wordplay.
'london area' becomes 'SE' (South East postcode).
'leaving' indicates named letters should be taken away.
'as a precaution' becomes 'in case' (just in case).
'incase' with 'se' taken out is 'INCA'.
'in' acts as a link.
(Other definitions for inca that I've seen before include "Pre-conquest Peruvian" , "Ruler of an old empire centred on Cuzco" , "Former emperor" , "Pre-conquest South American Indian" , "Original Peruvian" .)