Radioactive element blows up, causing confusion (5)
I believe the answer is:
snafu
'confusion' is the definition.
(I've seen this before)
'radioactive element blows up' is the wordplay.
'radioactive element' becomes 'U' (chemical symbol for uranium).
'blows' becomes 'fans' (fan can mean to blow air).
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'u'+'fans'='ufans'
'ufans' back-to-front is 'SNAFU'.
'causing' is the link.
(Other definitions for snafu that I've seen before include "chaos, as usual" , "Mess table" , "in a bad state" , "Chaotic state, mess" , "Slangy acronym meaning in total confusion" .)