Fly is twice tested, oddly (6)
I believe the answer is:
tsetse
'fly' is the definition.
(tsetse is a kind of fly)
'twice tested oddly' is the wordplay.
'twice' shows that letters should be duplicated.
'oddly' indicates alternate letters.
The alternating letters of 'tested' are 'tse'.
'tse' doubled is 'TSETSE'.
'is' acts as a link.
(Other definitions for tsetse that I've seen before include "Disease-carrying fly" , "carrier of disease" , "flies" , "Bloodsucking African fly" , "fly across Africa" .)