Unprincipled soldiers given test of will (9)
I believe the answer is:
reprobate
'unprincipled' is the definition.
(thesaurus)
'soldiers given test of will' is the wordplay.
'soldiers' becomes 're' (Royal Engineers).
'given' means one lot of letters go next to another.
'test of will' becomes 'probate' (test to validate a deceased person's will).
're'+'probate'='REPROBATE'
(Other definitions for reprobate that I've seen before include "Profligate" , "Person without moral scruples" , "rake" , "Ne'er-do-well" , "criminal" .)