Dictatorial American soldiers encased in concrete (11)
I believe the answer is:
magisterial
'dictatorial' is the definition.
(thesaurus)
'american soldiers encased in concrete' is the wordplay.
'american soldiers' becomes 'GIs' (GI is an informal term for a US soldier).
'encased in' means one lot of letters goes inside another.
'concrete' becomes 'material' (I can't justify this - if you can you should give a lot more credence to this answer).
'gis' put within 'material' is 'MAGISTERIAL'.
Can you help me to learn more?
(Other definitions for magisterial that I've seen before include "unlikely to be argued with" , "Commanding" , "enforcing law" .)