Like turnover or top biscuit (3)

'biscuit' is the definition.
I don't know anything about this answer so I can't judge whether it can be defined by this definition.

'like turnover or top' is the wordplay.
'like' becomes 'as'.
'turnover' says the letters should be written in reverse (the letters turn over).
'top' suggests taking the first letters (in a down clue, the first letter is top).
The initial letter of 'or' is 'o'.
'as' written backwards gives 'sa'.
'sa'+'o'='SAO'

Can you help me to learn more?