Visual aids for gymnasium's top girls (7)
I believe the answer is:
glasses
'visual aids' is the definition.
(I've seen this in another clue)
'gymnasium's top girls' is the wordplay.
'top' indicates taking the first letters (in a down clue, the first letter is top).
'girls' becomes 'lasses' (lass can mean a girl).
The initial letter of 'gymnasiums' is 'g'.
'g'+'lasses'='GLASSES'
'for' acts as a link.
(Other definitions for glasses that I've seen before include "They make girls unattractive to Parker" , "Spectacles; tumblers" , "Specs" , "Barnacles" , "are they full of alcohol?" .)