Sun radical at the bottom anchors 暂 in the time family: 'temporary' is a measure of duration. The same radical pulls in 时 time, 晨 dawn, 早 early - anything reckoned by where the sun is.
Top supplies the sound: zhǎn → zàn (initial drifted, rhyme intact). 斩's meaning of 'cutting' also carries faint semantic flavour - a temporary moment is a slice cut out of a longer span of time, a brief interruption.
