Greatest of all time secures large triumph (5)
I believe the answer is:
gloat
'triumph' is the definition.
('gloat' can be a synonym of 'triumph')
'greatest of all time secures large' is the wordplay.
'greatest of all time' becomes 'goat' (abbreviation often used in sport).
'secures' means one lot of letters goes inside another.
'large' becomes 'l'.
'goat' placed around 'l' is 'GLOAT'.
(Other definitions for gloat that I've seen before include "Be full of malicious delight" , "Express triumphant self-satisfaction" , "Look triumphant" , "Take malicious satisfaction" , "Exult maliciously" .)