Old words have unsettled dwarf (10)
I believe the answer is:
overshadow
'dwarf' is the definition.
(I know that dwarf can be written as overshadow)
'old words have unsettled' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'unsettled' indicates an anagram.
'words'+'have'='wordshave'
'wordshave' anagrammed gives 'vershadow'.
'o'+'vershadow'='OVERSHADOW'
(Other definitions for overshadow that I've seen before include "Cast gloom on" , "Dominate" , "Eclipse" , "Dwarf" .)