Left metal radical, the simplified left-side form of
金. It supplies the meaning:
钩 is a metal hook - fish hook, meat hook, hanger. The radical anchors a vast metals-and-tools family:
钉 (nail),
针 (needle),
锁 (lock),
链 (chain),
锤 (hammer),
铁 (iron). Anywhere you see
钅 on the left, expect a metal object or tool.