Sheepdog with extremes of rascality is mine (8)
I believe the answer is:
colliery
'is mine' is the definition.
(thesaurus)
'sheepdog with extremes of rascality' is the wordplay.
'sheepdog' becomes 'collie' (collie is a kind of sheepdog).
'with' says to put letters next to each other.
'extremes of' means to remove the middle letters.
'rascality' with its middle taken out is 'ry'.
'collie'+'ry'='COLLIERY'
(Other definitions for colliery that I've seen before include "Grimethorpe?" , "Coal mine" , "Where a coalminer goes to work" , "Mining complex" , "Coalmine and surrounding buildings" .)