Rock may be suitable for the foundation (5)
I believe the answer is:
stone
'rock' is the definition.
(I know that rock can be written as stone)
'suitable for the foundation' is the wordplay.
I cannot quite understand how this works, but
'for' could be 'to' (eg both can mean 'in the opinion of') and 'to' is found in the answer.
This explanation may well be incorrect...
'may be' acts as a link.
Can you help me to learn more?
(Other definitions for stone that I've seen before include "Hard lump of mineral matter" , "14 lb" , "See 19" , "Rock; fruit seed" , "diamond, say" .)