Hide, formerly found in California (7)
I believe the answer is:
conceal
'hide' is the definition.
(I know that hide can be written as conceal)
'formerly found in california' is the wordplay.
'formerly' becomes 'once' (synonyms).
'found in' is an insertion indicator.
'california' becomes 'cal' (short for California).
'once' put within 'cal' is 'CONCEAL'.
(Other definitions for conceal that I've seen before include "Hide, cover" , "'Hide, keep secret (7)'" , "Screen" , "Keep out of sight" , "hush up!" .)