The enemy seized the gap to attack, catching us completely off guard.
Tips
register
隙 is a classical/literary character. In modern speech it usually appears in compounds: 间隙 (gap/interval), 空隙 (gap/opening), 嫌隙 (rift/grudge), 裂隙 (fissure). Rarely used alone in modern colloquial speech.
memory
Picture the radical 阝(hill/mound) with 小 (small) — a small gap in the hillside.