One visiting California, the French Republican (6)
I believe the answer is:
caller
'one visiting' is the definition.
(someone calling in on someone else)
'california the french republican' is the wordplay.
'california' becomes 'cal' (short for California).
'the french' becomes 'le' ('the' in French).
'republican' becomes 'r' (abbreviation).
'cal'+'le'+'r'='CALLER'
(Other definitions for caller that I've seen before include "One visits" , "One paying a brief visit" , "Person on the line" , "Someone phoning" , "Visitor" .)