Crooked teeth -- gran's wanting a new brace (10)
I believe the answer is:
strengthen
'brace' is the definition.
(thesaurus)
'crooked teeth gran's wanting a new' is the wordplay.
'crooked' indicates an anagram.
'wanting' is a deletion indicator (to want something can mean to lack it or need it).
'new' becomes 'n' (**).
'teeth'+'grans'='teethgrans'
'teethgrans' with 'a' taken out is 'teethgrns'.
'teethgrns' is an anagram of 'strengthe'.
'strengthe'+'n'='STRENGTHEN'
(Other definitions for strengthen that I've seen before include "Brace" , "Bolster" , "Fortify" , "Toughen up" , "Make tough" .)