Part of an atom of gas probed by London university (7)
I believe the answer is:
nucleon
'part of an atom' is the definition.
('nucleon' is part of 'atom')
'gas probed by london university' is the wordplay.
'gas' becomes 'neon' (neon is a gas).
'probed by' is an insertion indicator (probe can mean to poke or prod into).
'london university' becomes 'ucl' (I can't justify this - if you can you should believe this answer much more).
'neon' enclosing 'ucl' is 'NUCLEON'.
'of' acts as a link.
Can you help me to learn more?
(Other definitions for nucleon that I've seen before include "fundamental constituent" , "Particle" .)