A mug's become a wise man
I believe the answer is:
magus
'a wise man' is the definition.
(I've seen this before)
'a mug's become' is the wordplay.
'become' indicates an anagram.
'a'+'mugs'='amugs'
'amugs' anagrammed gives 'MAGUS'.
(Other definitions for magus that I've seen before include "eg Melchior" , "astrologer" , "Magician, wise man" , "Gift-bearing Wise Man" , "Wizard" .)