Brown-clad soldier occupied a trench (3,2)
I believe the answer is:
dug in
'occupied a trench' is the definition.
Although both the answer and definition are past participle verbs, I don't see how one could define the other.
'brown-clad soldier' is the wordplay.
'brown' becomes 'dun' (dun is a kind of brown).
'clad' means one lot of letters goes inside another (clad can mean covered by).
'soldier' becomes 'GI' (GI can mean a US soldier).
'dun' enclosing 'gi' is 'DUG IN'.
Can you help me to learn more?
(Another definition for dug in that I've seen is " Firmly established".)