Gas about attempt to infiltrate navy (5)
I believe the answer is:
argon
'gas' is the definition.
(thesaurus)
'about attempt to infiltrate navy' is the wordplay.
'about' becomes 'a' (abbreviation in Chambers**).
'attempt' becomes 'go' (go is a kind of attempt).
'to infiltrate' means one lot of letters goes inside another.
'navy' becomes 'rn' (abbreviation for Royal Navy).
'go' put into 'rn' is 'rgon'.
'a'+'rgon'='ARGON'
(Other definitions for argon that I've seen before include "Gas, 1% of atmosphere" , "Gas used in electric lights" , "Noble gas, atomic number 18" , "rare gas" , "Inert gas in atmosphere" .)