Fool oddly missing king and knight for set (6)
I believe the answer is:
assign
'for set' is the definition.
(thesaurus)
'fool oddly missing king and knight' is the wordplay.
'fool' becomes 'ass' (both can mean a stupid person).
'oddly missing' means one should take alternating letters (letters in odd-numbered positions removed).
'and' means one lot of letters go next to another.
'knight' becomes 'n' (chess abbreviation).
The alternate letters of 'king' are 'ig'.
'ass'+'ig'+'n'='ASSIGN'
(Other definitions for assign that I've seen before include "Entrust or convey to" , "Allocate, a job say" , "Make over" , "Allot or ascribe" , "Allocate, appoint" .)