Headgear up in space (4)

I believe the answer is:

caps

I'm a little stuck... Click here to teach me more about this clue!

'headgear' is the definition.
(caps are a type of headgear)

I don't understand how the remainder of the clue works.

(Other definitions for caps that I've seen before include "Head-coverings" , "Coverings for heads or bottles" , "Coverings, for heads say" , "Crowns" , "Style of GBS writing" .)

I've seen this clue in the Evening Standard.

Want a hint initially instead of a full solution? Install my app