Fine giving us washing if in bag (4)

I believe the answer is:

thin

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

'fine' is the definition.
(I've seen this before)

'us washing if in bag' is the wordplay.
I cannot really understand how this works, but
'in' is within the answer.
This explanation may well be incorrect...

'giving' is the link.

Can you help me to learn more?

(Other definitions for thin that I've seen before include "Implausible" , "Sparse; not dense" , "Unconvincing" , "as 7 [LOAF] may be sliced" , "With little thickness" .)

I've seen this clue in The Telegraph.

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