The heart of the matter (7)
I believe the answer is:
nucleus
'the heart of the matter' is the definition.
(I've seen this before)
'matter' is the wordplay.
I cannot really understand how this works, but
'matter' could be 'eu' (Eu is an example) and 'eu' is found within the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for nucleus that I've seen before include "Core, centre" , "Central core of an atom" , "Central mass of a cell" , "hard core" , "Centre of growth" .)