Stylish cover for pastry that's lacking sides (7,3)

I believe the answer is:

porkpie hat

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'that's lacking sides' is the definition.
I can't tell whether this defines the answer.

'stylish cover for pastry' is the wordplay.
'stylish cover' becomes 'hat' (cover for one's head. I am not sure about the 'stylish' bit.).
'for' means one lot of letters go next to another (I've seen this in other clues).
'pastry' becomes 'pork pie' (pork pie is a kind of pastry).
'hat' after 'porkpie' is 'PORKPIE HAT'.

Can you help me to learn more?

I've seen this clue in The Irish Times.

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