Acknowledge successful strike to union rebel (6)
I believe the answer is:
touche
'acknowledge successful' is the definition.
I know nothing about this answer so I can't tell whether it can be defined by this definition.
'strike to union rebel' is the wordplay.
'strike' becomes 'touch' (touching is a kind of striking).
'to union rebel' becomes 'e' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'touch'+'e'='TOUCHE'
Can you help me to learn more?
(Other definitions for touche that I've seen before include "Acknowledgment of a hit in fencing" , "Acknowledgment of hit by fencer" , "cry from fencer" , "That hit home" , "Fencer's shout" .)