Real breakthrough when club's discarded (4)
I believe the answer is:
very
'real' is the definition.
(I know that real can be written as very)
I cannot understand how the rest of the clue works.
(Other definitions for very that I've seen before include "Actual; in a high degree" , "Ever so" , "type of light" , "light, perhaps" , "Extremely, in a high degree" .)