A social event among the fleet is a gas (5)
I believe the answer is:
radon
'gas' is the definition.
(I know that radon is a type of argonon)
'a social event among the fleet' is the wordplay.
'social event' becomes 'do' (do is a kind of social event).
'among' means one lot of letters goes inside another (inserted letters are placed among others).
'the fleet' becomes 'RN' (abbreviation for Royal Navy).
'a'+'do'='ado'
'ado' put inside 'rn' is 'RADON'.
'is a' is the link.
(Other definitions for radon that I've seen before include "Colourless gas, atomic number 86" , "Element, symbol Rn" , "hazardous substance" , "Naturally occurring inert gas" , "Gaseous element, Rn" .)