Lamp insert

BULB

It glows when electricity passes through its filament inside a light fixture.

The clue "Lamp insert" is a straightforward definition where "insert" hints at something placed inside a lamp. The answer is "BULB," which is the part you screw or fit into a lamp to produce light. There’s no wordplay here—it’s a direct reference to the common object that powers a lamp. Think of the glass bulb you’d find in a desk lamp or ceiling fixture.