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' indicates putting letters inside (inserted letters are secured inside).
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'victory' becomes 'v'.
'n'+'v'='nv'
'coalesce' going around 'nv' is 'CONVALESCE'.
'and' acts as a link.
(Other definitions for convalesce that I've seen before include "Improve" , "Recover after an illness" .)