Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
dēclārātīvus, a, um, adj. [declaro], serving for explanation, explanatory (postclass.): pars, App. Dogm. Plat. 3, p. 30 and 33; Mart. Cap. 4, § 341.
Adv.: dēclārā-tīvē, by way of explanation, Mart. Cap. 4, § 393.