Form spaces that many think are overcrowded (10)

I believe the answer is:

classrooms

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'many think are overcrowded' is the definition.
I can't judge whether this definition defines the answer.

'form spaces' is the wordplay.
'form' becomes 'class' (synonyms).
'spaces' becomes 'rooms' (room is a kind of space).
'class'+'rooms'='CLASSROOMS'

'that' is the link.

Can you help me to learn more?

(Other definitions for classrooms that I've seen before include "Places for lessons" , "instruction areas" .)

I've seen this clue in The Irish Times.

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