Underlying support (5)

I think it could be either:

Which length description are you looking for?:

basis

'underlying support' is the definition.
(I've seen this before)

This is the entire clue.

(Other definitions for basis that I've seen before include "Reason" , "grounds" , "Fundamental part" , "Foundation or support, or a main principle" , "Ground, foundation" .)

I've seen this clue in The Times.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.