John Sullivan's last joke's stuck in the distant past (4-3)

'the distant past' is the definition.
(I've seen this before)

'john sullivan's last joke's stuck in' is the wordplay.
'john' becomes 'loo' (both can mean a toilet).
'last' indicates one should take the final letters.
'joke' becomes 'gag' (a gag is a joke or witticism).
'stuck in' is an insertion indicator.
The last letter of 'sullivan' is 'n'.
'n'+'gag'='ngag'
'loo' enclosing 'ngag' is 'LONG-AGO'.