Essentially scrub before opening X-ray centre (4)
I believe the answer is:
crux
'centre' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
'essentially scrub before opening x-ray' is the wordplay.
'essentially' indicates the central letters.
'before' is a charade indicator (letters next to each other).
'opening x-ray' becomes 'x' (phonetic alphabet: alpha, bravo, charlie etc.. I am not sure about the 'opening' bit.).
The centre of 'scrub' is 'cru'.
'cru'+'x'='CRUX'
Can you help me to learn more?
(Other definitions for crux that I've seen before include "puzzle" , "Pivotal point - cross" , "Essential point in the matter or problem" , "Essence or gist" , "Core" .)