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 conceal)
'formerly seen in california' is the wordplay.
'formerly' becomes 'once' (synonyms).
'seen in' is an insertion indicator.
'california' becomes 'cal' (short for California).
'once' put into '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!" .)