Soldier breaking free is not yielding (5)
I believe the answer is:
rigid
'is not yielding' is the definition.
(both can mean inflexible)
'soldier breaking free' is the wordplay.
'soldier' becomes 'GI' (GI can mean a US soldier).
'breaking' is an insertion indicator.
'free' becomes 'rid' (synonyms).
'gi' going inside 'rid' is 'RIGID'.
(Other definitions for rigid that I've seen before include "Stiff, unmoving" , "Hard" , "Firmly fixed" , "Unable to bend" , "stern" .)