Met radical in Congress, which is explosive (6)
I believe the answer is:
semtex
'is explosive' is the definition.
(I've seen this before)
'met radical in congress' is the wordplay.
'radical' indicates an anagram.
'in' means one lot of letters goes inside another.
'congress' becomes 'sex'.
'met' with letters rearranged gives 'mte'.
'mte' put into 'sex' is 'SEMTEX'.
'which' is the link.
(Other definitions for semtex that I've seen before include "Plastic explosive used by terrorists" , "Explosive material used by terrorists" , "Type of plastic explosive" , "Explosive substance used by terrorists" , "Plastic explosive from the Czech Republic" .)