Caught thus, you’re taken by surprise (8)
I believe the answer is:
unawares
'by surprise' is the definition.
(thesaurus)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for unawares that I've seen before include "Unknowingly - by surprise" , "Be taken thus if off-guard" , "In ignorance" , "Without knowledge of the situation" , "New auras may be unexpected" .)