Word used by unprincipled genius (6)
I believe the answer is:
pledge
'word' is the definition.
(thesaurus)
'used by unprincipled genius' is the wordplay.
'used by' indicates the answer is hidden within the clue.
'PLEDGE' is hidden within 'unprincipled genius'.
(Other definitions for pledge that I've seen before include "Plight" , "Vow, affirm" , "make toast" , "Promise, word of honour" , "Solemn promise" .)