Boyish young ladies wearing sports skirts (7)
I believe the answer is:
gamines
'boyish young ladies' is the definition.
(a gamine is a woman with attractively boyish looks)
'wearing sports skirts' is the wordplay.
'wearing' becomes 'in' (wear in (and 'in' is fashionable)).
'sports' becomes 'games' (I've seen this before).
'skirts' means one lot of letters goes inside another.
'in' placed inside 'games' is 'GAMINES'.
(Other definitions for gamines that I've seen before include "Mischievous girls" , "Girls like boys" .)