Leading Irish nurse cares about some old Americans (5)
I believe the answer is:
incas
'old americans' is the definition.
(historical South American civilisation)
'leading irish nurse cares about some' is the wordplay.
'leading' indicates taking the first letters (the leading letter of the word).
The initial letters of 'irish nurse cares about some' is 'INCAS'.
(Other definitions for incas that I've seen before include "Old people coming from South America" , "Ancient inhabitants of Peru" , "Old South American royals" , "South Americans whose empire fell to Spain" , "People of ancient Peru" .)