Old lady gets cross over gnome (5)
I believe the answer is:
maxim
'gnome' is the definition.
(both can mean a saying)
'old lady gets cross over' is the wordplay.
'old lady' becomes 'ma'.
'gets' says to put letters next to each other.
'cross' becomes 'mix' (**).
'over' says the letters should be written backwards.
'mix' reversed gives 'xim'.
'ma'+'xim'='MAXIM'
(Other definitions for maxim that I've seen before include "Widely accepted saying" , "- Gorky, Russian writer" , "Pithy statement of a general truth" , "Principle-first machine gun" , "Saw - type of gun" .)