Crossword Genius

Spend time inside bank after door locks quietly (2,8)

Ross

I believe the answer is:

do porridge

I'm a little stuck... Click here to teach me more about this clue!

'bank after door locks quietly' is the definition.
I can't judge whether this defines the answer.

'spend time' is the wordplay.
'spend' becomes 'do' (doing is a kind of spending).
'time' becomes 'porridge' (I've seen this before).
'do'+'porridge'='DO PORRIDGE'

'inside' acts as a link.

Can you help me to learn more?

(Other definitions for do porridge that I've seen before include "detained" , "Get time" , "Spend time in prison (colloq.)" .)

I've seen this clue in The Times.
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