The dog on the line is mine (8)
I believe the answer is:
colliery
'is mine' is the definition.
(thesaurus)
'the dog on the line' is the wordplay.
'the dog' becomes 'collie' (collie is a kind of dog**).
'on' is a charade indicator (letters next to each other).
'the line' becomes 'ry' (abbreviation for railway).
'collie'+'ry'='COLLIERY'
(Other definitions for colliery that I've seen before include "Coal mine" , "Pit" , "Coal-mining establishment" , "Where a coalminer goes to work" , "Grimethorpe?" .)