Mark and Don trained raider (8)
I believe the answer is:
commando
'raider' is the definition.
(I have seen 'Military raider' mean 'commando' so perhaps 'raider' could also mean 'commando')
'mark and don trained' is the wordplay.
'mark' becomes 'comma' (punctuation mark).
'and' means one lot of letters go next to another.
'trained' is an anagram indicator (to train can mean to grow something into a shape).
'don' with letters rearranged gives 'ndo'.
'comma'+'ndo'='COMMANDO'
(Other definitions for commando that I've seen before include "shock troops" , "Specialist soldier" , "Soldier trained to carry out dangerous raids" , "Soldier trained for raiding" , "Royal Marine Corps soldier" .)