Mother dear hiding deserter from battle (8)
I believe the answer is:
marathon
'battle' is the definition.
(Battle of Marathon, 490 BC)
'mother dear hiding deserter' is the wordplay.
'mother' becomes 'ma' (Ma is another word for mother or mum).
'dear' becomes 'hon' (both are terms of endearment).
'hiding' is an insertion indicator.
'deserter' becomes 'rat' (both can mean a traitor).
'ma'+'hon'='mahon'
'mahon' enclosing 'rat' is 'MARATHON'.
'from' acts as a link.
(Other definitions for marathon that I've seen before include "Footrace of 26 miles 385 yards" , "Long and arduous" , "26-mile foot race" , "The longest foot race in sport" , "Footrace of approx 26 miles" .)