Top item California put inside bread roll (7,3)
I believe the answer is:
Here is my best explanation:
'top' is the definition.
(piece of headwear for one's top)
'item california put inside bread roll' is the subsidiary indication.
'item' becomes 'thing'.
'california' becomes 'ca'.
'put inside' is an insertion indicator.
'bread roll' becomes 'bap' (I've seen this before).
'thingca' inserted inside 'bap' is 'bathingcap'.
(Another definition for bathing cap that I've seen is "Swimmer's hair protector".)