auxiliary N3 uncommon politewritten kenjougo (humble)
〜でさしあげる — voiced allomorph of 〜てさしあげる
〜でさしあげる ・ でさしあげる
Builds on て形
A full written guide for 〜でさしあげる is on the way.
This point is mapped in the grammar tree, but its examples, formation table and usage notes haven't been written yet. The fastest way to learn it now is to meet it in real sentences inside Jengo.
See 〜でさしあげる in real sentences
Jengo shows 〜でさしあげる the way you actually meet it: inside real Japanese sentences, so it sticks instead of staying an abstract rule.
Study it in JengoSources Compiled with reference to A Dictionary of Basic Japanese Grammar. Editorial confidence: medium.