This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
The exact historical development of 聀 is uncertain. It visually combines 耳 (ear) and 戈 (halberd), but the original meaning and component relationship are unclear.
Since the meaning is uncertain, focus on the shape: an ear 耳 beside a halberd 戈. Perhaps it once described listening for danger or a military ear, but treat this as a visual placeholder.
For ショク, imagine a soldier with a halberd 戈 listening 耳 for a shock: shock -> ショク, and the ear catches the sound of battle.