Fight over old earth (3-2)
I believe the answer is:
set-to
'fight' is the definition.
(I've seen this before)
'over old earth' is the wordplay.
'over' says to put letters next to each other (in a down clue, some letters go over others).
'old' becomes 'o' (abbreviation).
'earth' becomes 'sett' (both can refer to an animal's underground home).
'o' after 'sett' is 'SET-TO'.
(Other definitions for set-to that I've seen before include "Fight, argument" , "Disagreement, fight" , "Ready - fight" , "Scrap" , "Altercation" .)