Earl's reformed around two thousand in prison (7)
I believe the answer is:
slammer
'prison' is the definition.
(thesaurus)
'earl's reformed around two thousand' is the wordplay.
'reformed' indicates an anagram.
'around' indicates putting letters inside.
'two thousand' becomes 'mm' (**).
'earls' is an anagram of 'slaer'.
'slaer' enclosing 'mm' is 'SLAMMER'.
'in' is the link.
(Other definitions for slammer that I've seen before include "Stir" , "I must be drunk in one" , "one leaping into crowd" , "Prison (slang)" , "Prison - critic?" .)