Boss, supposedly dead cross after returning to nick (8)
I believe the answer is:
doornail
'boss supposedly dead' is the definition.
(dead as a doornail - boss can mean a metal stud)
'cross after returning to nick' is the wordplay.
'cross' becomes 'rood' (term for a crucifix).
'after returning' says the letters should be written backwards.
'to nick' becomes 'nail' (both can mean to apprehend or arrest).
'rood' in reverse letter order is 'door'.
'door'+'nail'='DOORNAIL'
(Other definitions for doornail that I've seen before include "Stud" , "proverbially dead" , "Dead as a ...-very dead indeed" , "popularly believed dead" , "Dead as a ........ is dead without a doubt" .)