Item of headwear (8,3)
(Other definitions for baseball cap that I've seen before include "Sports headgear" , "What sportsman may assume" , "American sporting headgear" , "Long-peaked headgear" , "sportswear" .)
'item of headwear' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for cap that I've seen before include "Surpass with headwear" , "Upper limit; hat" , "Go with this in hand to be obsequious" , "Soft hat; crown" , "Peaked hat" .)