Japan working to secure one ally (4)
I believe the answer is:
join
'ally' is the definition.
(thesaurus)
'japan working to secure one' is the wordplay.
'japan' becomes 'j'.
'working' becomes 'on'.
'to secure' indicates putting letters inside.
'one' becomes 'i' (Roman numeral**).
'on' going around 'i' is 'oin'.
'j'+'oin'='JOIN'
(Other definitions for join that I've seen before include "Couple" , "Amalgamate" , "Enrol in" , "Associate, combine" , "ally" .)