Old t-shirt boy redesigned is savage (12)
I believe the answer is:
bloodthirsty
'is savage' is the definition.
(I've seen this before)
'old t-shirt boy redesigned' is the wordplay.
'redesigned' indicates an anagram.
'old'+'t'+'shirt'+'boy'='oldtshirtboy'
'oldtshirtboy' is an anagram of 'BLOODTHIRSTY'.
(Other definitions for bloodthirsty that I've seen before include "Eager to kill and maim" , "Desiring carnage" , "Murderous" , "Craving violence and slaughter" , "Depicting much violence" .)