Fuss as radioactive gas uncovered (3)
I believe the answer is:
ado
'fuss' is the definition.
(synonyms)
'radioactive gas uncovered' is the wordplay.
'radioactive gas' becomes 'radon' (I've seen this before).
'uncovered' says to take the centre.
The middle of 'radon' is 'ADO'.
'as' is the link.
(Other definitions for ado that I've seen before include "Fuss, bother" , "Nothing caused much around 1598" , "Fuss, difficulty" , "Rapid bustling commotion" , "flapping" .)