Great success in showing the flag (6,7)
I believe the answer is:
flying colours
'great success' is the definition.
'flying colours' can be an answer for 'success' (I have seen 'Conspicuous success' mean 'flying colours' so perhaps 'success' could also mean 'flying colours'). I am not certain of the 'great' bit.
'showing the flag' is the wordplay.
'showing' becomes 'flying' (I can't explain this - if you can you should believe this answer much more).
'the flag' becomes 'colours' (colours is a kind of flag).
'flying'+'colours'='FLYING COLOURS'
'in' acts as a link.
Can you help me to learn more?
(Other definitions for flying colours that I've seen before include "Conspicuous success" , "With which the triumphant succeed" , "Symbol of great success" .)