We with a self-confidence gaining new convert for hostile use (9)

'convert for hostile use' is the definition.
(to weaponise something is to make it a weapon)

'we with a self-confidence gaining new' is the wordplay.
'with' says to put letters next to each other.
'self-confidence' becomes 'poise' (I've seen this before).
'gaining' means one lot of letters goes inside another.
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'poise' going around 'n' is 'ponise'.
'we'+'a'+'ponise'='WEAPONISE'