Big girl struggling for breath? (6)

I believe the answer is:

bertha

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

'big girl' is the definition.
(I've seen this in another clue)

'struggling for breath?' is the wordplay.
'breath' anagrams to 'BERTHA'.
But, I am not sure how this is indicated.

Can you help me to learn more?

(Other definitions for bertha that I've seen before include "Woman's name" , "43-ton German howitzer, Big _____" , "Big - - , WW1 gun" , "Big girl" , "big woman?" .)

I've seen this clue in the Evening Standard.

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