New phrase in old record kept by Napoleonic marshal (7)
I believe the answer is:
neology
'new phrase' is the definition.
(new word or phrase)
'old record kept by napoleonic marshal' is the wordplay.
'old' becomes 'o' (abbreviation).
'record' becomes 'log' (logging is a kind of recording).
'kept by' means one lot of letters goes inside another.
'napoleonic marshal' becomes 'ney' (Michel Ney, French military commander).
'o'+'log'='olog'
'olog' going inside 'ney' is 'NEOLOGY'.
'in' acts as a link.
(Other definitions for neology that I've seen before include "The coining of new words - eg loony (anag)" , "Fresh word" , "Act of inventing a word or phrase" , "new term" .)