Sailors outside a party that’s a gas! (5)
I believe the answer is:
radon
'a gas' is the definition.
(radon is a kind of gas)
'sailors outside a party' is the wordplay.
'sailors' becomes 'rn' (abbreviation for Royal Navy).
'outside' means one lot of letters goes inside another.
'party' becomes 'do' (do is a kind of party).
'a'+'do'='ado'
'rn' placed around 'ado' is 'RADON'.
'that's' is the link.
(Other definitions for radon that I've seen before include "A radioactive gas, symbol Rn" , "Radioactive gas, linked to granite" , "hazardous substance" , "Colourless gas, atomic number 86" , "Naturally occurring inert gas" .)