Concept used in Madrid earlier (4)
I believe the answer is:
idea
'concept' is the definition.
(I've seen this before)
'in madrid earlier' is the wordplay.
'in' indicates a hidden word.
'IDEA' is hidden in the letters of 'madrid earlier'.
'used' acts as a link.
(Other definitions for idea that I've seen before include "Thought or notion (4)" , "Flash of inspiration?" , "Aim" , "Clue" , "Creative spark" .)