Threesome’s endless speed should secure United victory (7)
I believe the answer is:
triumph
'victory' is the definition.
('triumph' can be a synonym of 'victory')
'threesome's endless speed should secure united' is the wordplay.
'threesome' becomes 'trio' ('trio' can be a synonym of 'threesome').
'endless' means to remove the last letter.
'speed' becomes 'mph' (miles per hour).
'should secure' means one lot of letters goes inside another.
'united' becomes 'u'.
'trio' with its final letter removed is 'tri'.
'tri'+'mph'='trimph'
'trimph' placed around 'u' is 'TRIUMPH'.
(Other definitions for triumph that I've seen before include "Victory - jubilation" , "hit" , "Exult in victory" , "Great victory or achievement" , "Victory, great success" .)