A baggy dress? (3,4)

I believe the answer is:

tea gown

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

'a baggy dress?' is the definition.
'tea gown' can be an answer for 'dress?' (tea gown is a kind of dress). I am unsure of the 'a baggy' bit.

'baggy dress?' is the wordplay.
'baggy' becomes 'tea' (I can't explain this - if you can you should believe this answer much more).
'dress?' becomes 'gown' (gown is a kind of dress).
'tea'+'gown'='TEA-GOWN'

Can you help me to learn more?

(Other definitions for tea gown that I've seen before include "Loose garment" , "party dress" .)

I've seen this clue in the Evening Standard.

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