Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
crĕpuscŭlum, i, n. [creper], twilight, dusk (cf. Varr. L. L. 6, § 5 Müll.); and in partic., evening twilight, the dusk of the evening (opp. diluculum, the morning twilight, dawn; poet. or in post-Aug. prose).