Brother turning up with companion grabbed by murderous twin (8)
I believe the answer is:
'brother' is the definition.
(Capuchin is a type of monk)
'turning up with companion grabbed by murderous twin' is the wordplay.
'turning' says the letters should be written in reverse.
'with' says to put letters next to each other.
'companion' becomes 'ch' (Companion of Honour).
'grabbed by' indicates putting letters inside.
'murderous twin' becomes 'Cain' (Cain killed Abel - some say they were twins).
'up' written backwards gives 'pu'.
'puch' inserted into 'cain' is 'CAPUCHIN'.
(Other definitions for capuchin that I've seen before include "Monkey -- or hooded cloak", "Friar; cloak and hood", "Religious person", "Monkey with a head like a monk's cowl", "China cup for kind of friar".)