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