Static charged particle going about small entry (9)
I believe the answer is:
insertion
'entry' is the definition.
(thesaurus)
'static charged particle going about small' is the wordplay.
'static' becomes 'inert' (both can mean not moving).
'charged particle' becomes 'ion' (I've seen this before).
'going about' means one lot of letters goes inside another.
'small' becomes 's' (abbreviation - e.g. clothes size).
'inert'+'ion'='inertion'
'inertion' enclosing 's' is 'INSERTION'.
(Other definitions for insertion that I've seen before include "implant" , "being a word, eg, added" , "Act of putting inside" , "Addition" .)