Retreat after slaughter associated with old battle (8)
I believe the answer is:
culloden
'battle' is the definition.
(I've seen this before)
'retreat after slaughter associated with old' is the wordplay.
'retreat' becomes 'den' (both can mean a lair or hideout).
'after' means one lot of letters go next to another.
'slaughter' becomes 'cull' (I have seen 'Selective slaughter ' mean 'cull' so perhaps 'slaughter' could also mean 'cull').
'associated with' says to put letters next to each other.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'cull'+'o'='cullo'
'den' put after 'cullo' is 'CULLODEN'.
(Other definitions for culloden that I've seen before include "Seventeen fourty six Jacobite defeat" , "Last battle on British soil" , "Asian aromatic oil" , "Butcher's success" , "Moor near Inverness" .)