Source of corruption of fat union boss (12)
I believe the answer is:
fountainhead
'source' is the definition.
(fountainhead is a kind of source)
'corruption of fat union boss' is the wordplay.
'corruption of' is an anagram indicator.
'boss' becomes 'head' (synonyms).
'fat'+'union'='fatunion'
'fatunion' anagrammed gives 'fountain'.
'fountain'+'head'='FOUNTAINHEAD'
'of' is the link.
(Other definitions for fountainhead that I've seen before include "Original source" , "spring" , "Chief source" .)