Trousers extending to the knee or just below, worn for riding, mountaineering etc (8)
I believe the answer is:
breeches
(Other definitions for breeches that I've seen before include "Trousers worn with stockings" , "Short trousers" , "Trousers reaching to just below the knee" , "Trousers ending just below the knee" , "Riding trousers" .)