Clue: Hospital volunteers seen in many places in Mexico (5)
Answer: SANTA
Explanation: Definition: seen in many places in Mexico
SAN (short for "sanitarium") + T.A. ("volunteers") 
Santa is seen at the start of many Mexican place names , Santa Cruz, Santa Catarina etc, although my reseacrch shows that it's probably just as (if not more) prolific in California and Newe Mexico.