Last word by competent subject (8)
I believe the answer is:
amenable
'subject' is the definition.
(thesaurus)
'last word by competent' is the wordplay.
'last word' becomes 'amen' (I've seen this before).
'by' says to put letters next to each other.
'competent' becomes 'able' ('able' can be similar in meaning to 'competent').
'amen'+'able'='AMENABLE'
(Other definitions for amenable that I've seen before include "Nameable as being willing to be controlled" , "Nameable as being subject to the law" , "Prepared to fall in with others' plans" , "Easily influenced" , "Open to suggestions" .)