Small child pinching King's headdress (5)
I believe the answer is:
mitre
'headdress' is the definition.
(I know that mitre is a type of headdress)
'small child pinching king's' is the wordplay.
'small child' becomes 'mite' (informal term).
'pinching' means one lot of letters goes inside another (taking inside).
'king' becomes 'R' (abbreviation for rex, king in Latin).
'mite' placed around 'r' is 'MITRE'.
(Other definitions for mitre that I've seen before include "Sort of joint" , "See boss gear up" , "that goes over a cleric's head?" , "Episcopal headdress" , "Remit (anag.)" .)