1. To lie in a position directly beneath.
    A stratum of clay underlies the surface gravel.
  2. To lie under or beneath.
  3. To serve as a basis of; form the foundation of.
    a doctrine underlying a theory
  4. To be subject to; be liable to answer, as a charge or challenge.
  5. To underlay.

The above text is a snippet from Wiktionary: underlie
and as such is available under the Creative Commons Attribution/Share-Alike License.

Need help with a clue?
Try your search in the crossword dictionary!