Like a slab forming a foundation (5)
I believe the answer is:
basal
'forming a foundation' is the definition.
(I've seen this before)
'like a slab' is the wordplay.
'a slab' anagrams to 'BASAL'.
Though, I'm unsure how the anagram is indicated.
Can you help me to learn more?
(Other definitions for basal that I've seen before include "At the lowest level" , "forming a foundation" , "Fundamental" , "bottom layer perhaps" .)