The phrase "slenderly provided for" is certainly rare and/or obsolete. There are only 9 hits in the amazon.com database, pretty much all from classical authors (see selection below). I agree with Peter that the phrase is not worth keeping in LEO, principally because it can be understood perfectly by a translation of its constituent parts, i.e. it is not an idiom. However, to allow successful translation of phrases such as this one LEO should add translations for the figurative senses of the adjective "slender", as these are completely absent at the moment. For example, Collins-Pons lists the following meanings:
resources, income: knapp, mager
chance, hope: schwach, gering
excuse, profit margin: dürftig, schwach
lead, majority: knapp, gering
Tom Jones by Henry Fielding, pg. 53
It is no wonder that in an age when this kind of merit is so little in fashion, and so slenderly provided for, persons possessed of it should very eagerly flock to a place where they were sure of being received with great complaisance;
Samuel Johnson: The Major Works by Donald Greene
This is not the description of a cruel climate, yet the dark months are here a time of great distress, because the summer ca do little more than feed itself, and winter comes with its cold and it scarcity upon families very slenderly provided.
Waverley by Walter Scott, pg. 118
By an old settlement, almost all the landed estates of the Baron went, after his death, to a distant relation; and it was supposed that Miss Bradwardine would remain but slenderly provided for, as the good gentleman's cash matters had been too long under the exclusive charge of Bailie Macwheeble, to admit of any great expectations from his personal succession.