underlay has 3 distinct synonyms spread across 1 of its 4 senses. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. WordNet records no antonym for it — antonymy is a relation between word forms rather than concepts, so it is deliberately sparse.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
raise or support (the level of printing) by inserting a piece of paper or cardboard under the type
“underlay the plate”
put (something) under or beneath
“They underlaid the shingles with roofing paper”
provide with a base, support, lining, or backing
“underlay the boards with joists”
Synonyms, antonyms and the broader and narrower relations for underlay come from Princeton University’s WordNet. Full sources and licences.