Child carrying king's headgear (5)
I believe the answer is:
mitre
'headgear' is the definition.
(mitre is a kind of headgear)
'child carrying king's' is the wordplay.
'child' becomes 'mite' (I've seen this in other clues).
'carrying' indicates putting letters inside.
'king' becomes 'R' (abbreviation for rex, king in Latin).
'mite' enclosing 'r' is 'MITRE'.
(Other definitions for mitre that I've seen before include "Bishop's cap - wood joint" , "in which one might find a bishop" , "episcopal head may come under this" , "bishop's head?" , "See boss gear up" .)