Reversal makes uranium go off (1-4)
I believe the answer is:
u-turn
'reversal' is the definition.
(u-turn is a kind of reversal)
'uranium go off' is the wordplay.
'uranium' becomes 'U' (U is the chemical symbol for uranium).
'go off' becomes 'turn' (both can mean to start to rot).
'u'+'turn'='U-TURN'
'makes' acts as a link.
(Other definitions for u-turn that I've seen before include "Sudden complete change of plan" , "An about face, complete change of mind" , "Embarrassing reversal" , "Complete reversal of direction or policy" , "Complete change of direction" .)