London university splitting gas particle (7)
I believe the answer is:
nucleon
'particle' is the definition.
(nucleon is a kind of particle)
'london university splitting gas' is the wordplay.
'london university' becomes 'ucl' (I can't justify this - if you can you should believe this answer much more).
'splitting' means one lot of letters goes inside another (some letters split a word into two).
'gas' becomes 'neon' (neon is a gas).
'ucl' placed into 'neon' is 'NUCLEON'.
Can you help me to learn more?
(Other definitions for nucleon that I've seen before include "fundamental constituent" , "Particle" .)