Great success in showing the flag (6,7)
I believe the answer is:
'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).
'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" .)