Hide as soon as having entered California (7)
I believe the answer is:
conceal
'hide' is the definition.
(I know that hide can be written as conceal)
'as soon as having entered california' is the wordplay.
'as soon as' becomes 'once' (once can mean immediately after).
'having entered' indicates putting letters inside.
'california' becomes 'cal' (short for California).
'once' inserted into 'cal' is 'CONCEAL'.
(Other definitions for conceal that I've seen before include "hush up!" , "Hide, cover" , "Keep out of sight" , "Screen" , "'Hide, keep secret (7)'" .)