A foundation that's humble (5)
I believe the answer is:
abase
'humble' is the definition.
('abase' can be a synonym of 'humble')
'a foundation' is the wordplay.
'foundation' becomes 'base' ('base' can be a synonym of 'foundation'**).
'a'+'base'='ABASE'
'that's' is the link.
(Other definitions for abase that I've seen before include "Humble (oneself)" , "Belittle, degrade" , "Humiliate" , "put to shame" , "what 11 [REPRIMANDS] are meant to do" .)