Type of tie (4)
(Other definitions for ascot that I've seen before include "Where ladies wear new hats" , "Racecourse, home to the Gold Cup since 1807" , "sporting venue" , "Royal Berkshire race-meeting (and fashion show!)" , "Racecourse and cravat" .)
'type of tie' is the definition.
(I've seen this in another clue)
This is all the clue.