Rally and unite to secure new victory (10)
I believe the answer is:
convalesce
'rally' is the definition.
(thesaurus)
'unite to secure new victory' is the wordplay.
'unite' becomes 'coalesce' (coalescing is a kind of uniting).
'to secure' means one lot of letters goes inside another (inserted letters are secured inside).
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'victory' becomes 'v'.
'n'+'v'='nv'
'coalesce' enclosing 'nv' is 'CONVALESCE'.
'and' is the link.
(Other definitions for convalesce that I've seen before include "Improve" , "Recover after an illness" .)