Paving slab (9)
'paving slab' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for flagstone that I've seen before include "Fine-textured rock" , "One walked on" , "Pavior made from Teflon gas" , "Paving slab" , "It'll lay (some say) at ground level" .)
(Other definitions for flag that I've seen before include "National standard" , "Emblematic rectangular piece of fabric" , "what Amundsen planted" , "Iris - paving slab" , "sink" .)