Little Boy Blue's triumph (7)
I believe the answer is:
victory
'triumph' is the definition.
('victory' can be a synonym of 'triumph')
'little boy blue's' is the wordplay.
'little boy' becomes 'vic' (short for Victor).
'blue' becomes 'tory' (I've seen this in other clues).
'vic'+'tory'='VICTORY'
(Other definitions for victory that I've seen before include "Success in a contest" , "See 8" , "Triumph" , "Success against an opponent" , "old naval vessel" .)