Chalice man split unthinking (10)
I believe the answer is:
mechanical
'unthinking' is the definition.
(thesaurus)
'chalice man split' is the wordplay.
'split' indicates an anagram (letters split up and reassembled).
'chalice'+'man'='chaliceman'
'chaliceman' anagrammed gives 'MECHANICAL'.
(Other definitions for mechanical that I've seen before include "Machine-like" , "Lacking thought or spontaneity" , "Robotic" , "Bottom perhaps" , "Automatic" .)