What the female blowing the whistle was wearing? (5,5)

I believe the answer is:

grass skirt

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'blowing the whistle was wearing?' is the definition.
I can't judge whether this defines the answer.

'what the female' is the wordplay.
'what' becomes 'grass' (I can't justify this - if you can you should give a lot more credence to this answer).
'the female' becomes 'skirt' (skirt is a kind of female).
'grass'+'skirt'='GRASS SKIRT'

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I've seen this clue in the Evening Standard.

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