Make Seb out to be thick (6)
I believe the answer is:
obtuse
'thick' is the definition.
(thesaurus)
'make seb out' is the wordplay.
'make' indicates an anagram.
'seb'+'out'='sebout'
'sebout' is an anagram of 'OBTUSE'.
'to be' acts as a link.
(Other definitions for obtuse that I've seen before include "Angle between 90 and 180 degrees" , "Lacking in insight" , "Dull, non-perceptive" , "Angle greater than 90 degrees" , "Thick, blunt" .)