He sits around producing a dissertation (6)
I believe the answer is:
thesis
'a dissertation' is the definition.
('thesis' can be a synonym of 'dissertation')
'he sits around' is the wordplay.
'around' indicates an anagram.
'he'+'sits'='hesits'
'hesits' is an anagram of 'THESIS'.
'producing' is the link.
(Other definitions for thesis that I've seen before include "Learned dissertation" , "writing following research" , "Essay putting forward a particular view" , "Treatise advancing a new point of view resulting from research" , "theory" .)