Knickerbockers (4,5)
I believe the answer is:
plus fours
'knickerbockers' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for plus fours that I've seen before include "leisure wear" , "Sportswear" , "Baggy sports trousers" , "golfer's wear" , "Bags" .)