A party held by sailors: it's a gas (5)
I believe the answer is:
radon
'a gas' is the definition.
(radon is a kind of gas)
'a party held by sailors' is the wordplay.
'party' becomes 'do' (do is a kind of party).
'held by' means one lot of letters goes inside another.
'sailors' becomes 'rn' (abbreviation for Royal Navy).
'a'+'do'='ado'
'ado' going within 'rn' is 'RADON'.
'it's' acts as a link.
(Other definitions for radon that I've seen before include "gets no reaction" , "Colourless gas, atomic number 86" , "Inert gas that occurs naturally" , "hazardous substance" , "Inert gas - adorn (anag)" .)