Fuss in Royal Navy making radioactive gas (5)
I believe the answer is:
radon
'radioactive gas' is the definition.
(I've seen this before)
'fuss in royal navy' is the wordplay.
'fuss' becomes 'ado' (synonyms).
'in' indicates putting letters inside.
'royal navy' becomes 'RN' (abbreviation for Royal Navy).
'ado' placed inside 'rn' is 'RADON'.
'making' acts as a link.
(Other definitions for radon that I've seen before include "gets no reaction" , "Inert gas that occurs naturally" , "Element, symbol Rn" , "Natural gas considered a hazard to health" , "A radioactive gas, symbol Rn" .)