Soldier is repeatedly besetting the opposition (10)
I believe the answer is:
antithesis
'opposition' is the definition.
(I know that antithesis is a type of opposition)
'soldier is repeatedly besetting the' is the wordplay.
'soldier' becomes 'ant' (soldier is a type of ant).
'repeatedly' indicates the doubling of some letters.
'besetting' indicates putting letters inside.
'is' duplicated is 'isis'.
'isis' enclosing 'the' is 'ithesis'.
'ant'+'ithesis'='ANTITHESIS'
(Other definitions for antithesis that I've seen before include "Opposite; contrasting idea" , "Contrast between two things" , "Direct opposite" , "I oppose the suggestion" , "Reverse" .)