arrow in one's quiver

have an arrow (left) in one's quiver - иметь козырь в запасе; неиспользованное средство, дополнительное преимущество, оставленное про запас

Meaning: have a number of resources or strategies that one can follow

The ability of my boss with languages was another arrow in his quiver (an important resource) when he had to deal with foreign companies.

See also [ace in the hole ]
[туз в рукаве]
[two strings to one's bow]