Prefix with thesis (4)
I believe the answer is:
anti
'prefix with thesis' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for anti that I've seen before include "An opponent - opposed to party policy, etc" , "Against, informally" , "on the other side" , "Naysayer" , "Prefix meaniing against" .)