The authentic side of Madrid (4)
I believe the answer is:
real
'the authentic' is the definition.
(I've seen this before)
'side of madrid' is the wordplay.
'side' becomes 'r' (abbreviation for right).
'of madrid' becomes 'eal' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'r'+'eal'='REAL'
Can you help me to learn more?
(Other definitions for real that I've seen before include "Actual; Brazilian currency" , "Authentic" , "Existing, not imaginery" , "Actually existing" , "Genuine - old Spanish coin" .)