Hide on one occasion in California (7)
I believe the answer is:
conceal
'hide' is the definition.
(synonyms)
'on one occasion in california' is the wordplay.
'on one occasion' becomes 'once' (synonyms).
'in' is an insertion indicator.
'california' becomes 'cal' (short for California).
'once' going into 'cal' is 'CONCEAL'.
(Other definitions for conceal that I've seen before include "hush up!" , "Hide, cover" , "Place out of sight" , "'Hide, keep secret (7)'" , "Keep out of sight" .)