Girl in form caught out (4)

I believe the answer is:

lass

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'girl' is the definition.
(lass is a kind of girl)

I don't understand how the remainder of the clue works.

(Other definitions for lass that I've seen before include "Scottish girl" , "Miss" , "Sweet resident of Richmond Hill?" , "Young woman (-. Eng.)" , "Familiar form of address for female" .)

I've seen this clue in The Guardian and The Telegraph.

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