Old people as a precaution finishing early (5)
I believe the answer is:
incas
'old people' is the definition.
(people of the Inca Empire)
'as a precaution finishing early' is the wordplay.
'as a precaution' becomes 'in case' (just in case).
'finishing early' means to remove the last letter (the word finishes earlier than normal).
'incase' with its last letter removed is 'INCAS'.
(Other definitions for incas that I've seen before include "Old Peruvians" , "Ancient Peruvians" , "Old South American royals" , "Peruvian natives" , "People from Andes who settled in Peru" .)