wick, marrow, staple (for stapler)...

padding, lining, foundation

core wire, conductor

continued alertness, unrelaxed alertness, remaining on one's guard...

eccentricity (in mechanics, electronics, etc.)

(lamp) wick

(lamp) wick

thick obi sash, sash padding

thinning (buds and branches)

spare pencil lead, pen refill (cartridge), replacement wick (e.g. of a lantern)

collar core (stiff fabric, etc. inside a collar)

dead center

center of a flower (centre), pistils and stamens

pencil lead case (for a mechanical pencil)

cardboard tube

lead pointer (for a clutch pencil), lead sharpener

candle wick trimmer, snuffer, douter

spare pencil lead, pen refill (cartridge), replacement wick (e.g. of a lantern)

lamp wick

mechanical pencil lead

dead center (of a baseball bat), sweet spot

mentally strong, strong-willed

to get to the heart of the matter, to get to the core, to hit (a ball) with the sweet spot (of a golf club...

toilet drain outlet

apple core, core of an apple

apple core, core of an apple

staple (for a stapler)

lead of a pencil

apple core, core of an apple

wick of a candle