Centre of an atom (7)
![Ross](/clue/static/ross-mug.min.png)
I believe the answer is:
nucleus
'centre of an atom' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for nucleus that I've seen before include "Atomic core" , "basis for development" , "Centre of an atom" , "central component" , "part of comet's head" .)