Foundation stone in Blenheim sat askew (13)
I believe the answer is:
establishment
'foundation' is the definition.
(thesaurus)
'stone in blenheim sat askew' is the wordplay.
'stone' becomes 'st'.
'in' means one lot of letters goes inside another.
'askew' is an anagram indicator.
'blenheim'+'sat'='blenheimsat'
'blenheimsat' with letters rearranged gives 'eablishment'.
'st' put inside 'eablishment' is 'ESTABLISHMENT'.
(Other definitions for establishment that I've seen before include "Foundation" , "powers that be" , "Permanent staff; business; household" , "Act of setting up" , "Formation" .)