Large prison officers failing to open smallish rooms (7)

I believe the answer is:

larders

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

'smallish rooms' is the definition.
'larders' can be an answer for 'rooms' (larder is a kind of room). I am unsure of the 'smallish' bit.

'large prison officers failing to open' is the wordplay.
'large' becomes 'L' (eg in clothes sizes).
'prison officers' becomes 'warders' (I've seen this before).
'failing to open' means to remove the first letter (the opening letter is removed).
'warders' with its initial letter taken off is 'arders'.
'l'+'arders'='LARDERS'

Can you help me to learn more?

(Other definitions for larders that I've seen before include "Food stores" , "Small food storerooms" , "Pantries" .)

I've seen this clue in The Everyman puzzle in The Observer.

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