Trained soldiers repeatedly left deserts arid (5)
I believe the answer is:
dried
'arid' is the definition.
Both the definition and answer are adjectives. Perhaps you can see an association between them that I don't see?
I don't understand the rest of the clue.
(Other definitions for dried that I've seen before include "Desiccated" , "Parched" , "Preserved, in a way" , "With moisture removed" , "dehydrated" .)