Disguise formerly seen in California (7)
I believe the answer is:
conceal
'disguise' is the definition.
(I know that disguise is a more specific form of the action conceal)
'formerly seen in california' is the wordplay.
'formerly' becomes 'once' (synonyms).
'seen in' indicates putting letters inside.
'california' becomes 'cal' (short for California).
'once' put inside 'cal' is 'CONCEAL'.
(Other definitions for conceal that I've seen before include "Place out of sight" , "Screen" , "Keep out of sight" , "'Hide, keep secret (7)'" , "hush up!" .)