Just ten in this new form (5)
I believe the answer is:
sixth
'just' is the definition.
Both the answer and definition are adjectives. Maybe there's an association between them I don't understand?
'ten in this new form' is the wordplay.
'ten' becomes 'x' (roman numeral).
'in' is an insertion indicator.
'new form' indicates an anagram (I've seen 'new form of' mean this).
'this' is an anagram of 'sith'.
'x' put within 'sith' is 'SIXTH'.
Can you help me to learn more?
(Other definitions for sixth that I've seen before include "Beaten by five" , "fraction" , "ordinal" , "Musical interval from A to F, say" .)