Helmet -- it's not this being worn by one knight (3,3)
I believe the answer is:
tin hat
'helmet' is the definition.
(tin hat is a kind of helmet)
'not this being worn by one knight' is the wordplay.
'not this' becomes 'that'.
'being worn by' is an insertion indicator (I've seen this in other clues).
'one' becomes 'i' (Roman numeral).
'knight' becomes 'n' (chess).
'i'+'n'='in'
'that' going around 'in' is 'TIN HAT'.
'it's' acts as a link.
(Other definitions for tin hat that I've seen before include "Finish it off to put the protective headgear on it" , "Protective helmet once worn by constuction and mine workers" , "Steel helmet (informal)" , "Headgear, worn for protection" , "Soldier's helmet" .)