Blithe man's set to ruin business (13)
I believe the answer is:
establishment
'business' is the definition.
(I've seen this before)
'blithe man's set to ruin' is the wordplay.
'to ruin' indicates anagramming the letters.
'blithe'+'mans'+'set'='blithemansset'
'blithemansset' is an anagram of 'ESTABLISHMENT'.
(Other definitions for establishment that I've seen before include "Those in power" , "Action of setting up on a permanent basis" , "Institutionalized authority" , "system" , "being incorporated in this" .)